A couple of points.
First thing. Make your base solid and secure ie. grounding within the earth. This solid base also applies to action and knowledge. Are you practicing magic and ritual in an instinctive way, or have you been trained to understand exactly what you are doing, step by step?
Many people do find that they have a natural talent at dealing with (raising, projecting and receiving) energy. Sometimes the energy is powerful, sometimes it is not. If you understand exactly what you are doing when raising, projecting and receiving energy, you have more chance of manipulating it with your will, rather than just pulling energy.
What are the energy sources you are pulling from? Personally, I find that there is more strength / power available if the energy source is merged with and gently tapped in to rather than pulled.
That said, in training witches, I always advise that raising power for spellwork should be done by actually raising the energy in a kinetic way, rather than using one's own energy. We also always do energy raising in a protective circle to hold the energy in, otherwise one just continually raises energy without building it up and holding it for use in the spell. In circle casting we tend to merge with the energies of the earth and let them flow through us, using our own energy to direct the flow into the creation of the circle. The circle is then sealed and released to live a part from our will. At the end of the ritual, we don't usually take the circle back up, but by our will release it to merge back with the energies it came from. Not all paths have this take on it though.
You said above that you have monitored your own energy. Have you tried to doing the same ritual, at the same time for a whole cycle or month or something and writing the results in a journal? This is a really good way to observe fluctuations (as well as improve one's skills). It needs to be exactly the same ritual though and it is also useful to have a meditation section within it - so you can understand the ritual you are performing in a different way.
Something like the Lesser Banishing Ritual Of The Pentagram can work well for this. Meditate whilst standing within the six-rayed star. Maybe you have some kind of ritual that is used in your path that will work in this way? Also a good idea to create a sacred space as you can then observe (and practice) your reaction to it.
Also a good idea to perform some kind of balancing ritual regularly. We do a lot of elemental balancing rituals, once one is balanced then you can observe / feel / hear / smell etc., what is coming in and what is going out.
A lot of magic is not about the amount of power one can generate, harness, pull, project. It is about the will and intent and the balance of both. Sometimes a little stroke of energy is all that is required, if the will is strong and the intent is clear then everything clicks into place.
I believe that emotion play a very large role in our magical lives. That is why I put such big emphasis, in my own practice, to learn how not only to identify, but to control my outward emotions. Emotions have energy, as everything else does, and this energy can indeed play a big part in our spell work.
When someone is depressed, it is said that they may feel drained of energy, that they would rather sit on a couch and lounge about because they dont have the energy to do anything. This is the same with magic. If we are in a "magical depression" then our magic is effected. It could be weak, or ineffective. This magical depression could be caused by a lot of things. If one is bored with their magical lives it could cause your magic to be ineffectual. If one was in a coven, and there was discord in the coven because of two or more unhappy parties, this could also cause a magical depression. The mundane world could also, most definitely, cause a magical depression. Problems at the work place, at home, in a relationship, or family matters could influence your emotions in a way that it would effect your magical workings. There is a reason that meditation is stressed before magical workings. We need to shuck off our daily, mundane lives for our magic to be effective. The emotions and high stress of the mundane world wil effect our emotions and our concentration in a such a way that our magic is effected.
The past also plays a huge part in how our magic works. We have all been through some experience in our lives that have dramatically altered our lives, our perspective of the world, and our perspective of people. You would have to be a hermit from birth for the previous to not have happened. These experiences in our lives can alter our emotional balances (don't think I am calling everyone crazy, emotional imbalances are completely normal), and when our emotions are altered, our magic, and, indeed, our magical abilities, are altered.
In conclusion, fluctuations in our magical lives, and our magical abilities, are normal, they should be expected. Think of them as a practice, and experience, that must be worked through to become a better, witch, magician, sorcerer, etc. etc. As for working through them, each situation is different, and though I could offer you stories and my own experiences of my own past, of my own strive to get past these fluctuations, I could not tell you how to fix your own, or to work through your own, because it is your experience to experience, your own fluctuation, to work through.
As a runner, I can run under the same conditions (weather, time of day, course, same shoes, same clothes, same amount of sleep the night before, same diet during the day, etc.) and have totally different paces. Sometimes you're just not on your game as much as other times (or, conversely, sometimes you're having the run of your life). Why are some athletes able to be in the zone sometimes and not others? Who knows... but it happens to the best of 'em.
Aside from that, Gabrielle has some excellent points--especially the last paragraph. :)