I have the exact same problem!
I had been teaching myself how to play the flute using books and on line videos so I do not have a teacher to guide me. I am a fairly advanced piano player so reading the manuscript and fingering are not difficult for me at all.
My main problem, and source of frustration is that my tone can be absolutely wonderful at times, and at other times, dreadful. I came across a post from someone saying that her tone was very inconsistent, exactly the same as me. I thought it was one of those things that every beginner gets, and if I practice enough, it will become consistent.
It took me a long time to realise that immediately or soon after a meal, it can be breakfast, lunch or dinner, my tone is wonderful. A few hours later, even though I do not feel hungry or hypoglycaemic, my tone is dreadful. Therefore, lack of energy and hypoglycaemia is unlikely to be the reason for my inconsistency.
I have come up with these 2 theories: When my stomach is full, it may have 2 effects which improves my tone:
1. The full stomach creates pressure within my abdomen and helps my diaphragm to expel air. I watched a you tube video saying that I should try and expel air at the bottom of my lungs first, as the air is warmer. I don't know whether this is possible, but ever since I used the warm air technique (by pretending to say "Hoo" , similar to what you do when you breath over your glasses with your mouth to get moisture on them to clean them), my tone has improved dramatically, and I can get the whole flute to vibrate all the time to produce a rich tone.
2. Eating diverts blood to the gut, and it warms my abdomen which transfer the warmth upwards across my diaphragm to warm up the air in my lungs and increase this warm air effect.
These are just my theories, I wonder if anyone has any thoughts on these.
Is the "warm air" technique well know amongst flutist? It did wonders to my tone.