Help with my flute... Intonation problem...?

Question:

Help with my flute... Intonation problem...?

This is my second year playing the flute in my school band. I've had feedback that when I'm holding long notes, I tend to go flat towards the end. I've tried my best to use my diaphragm to support my note and not play so loud but it doesn't seem to work.

Also, sometimes even though I don't touch my headjoint, the first time I play a note, I may be sharp. However the second time I play it, it may become flat. Why?

My last question... I can't reach the lowest note on my flute, C. However, when I try playing that note on another flute, I'm able to play it easily. Is there something I can do to my own flute to fix that problem?

Additional Details

7 months ago
Hey thanks for all your suggestions and compliments! I really appreciate it. I will definitely take note of all your advice.

Regarding my flute, it's an Altus but I don't know which model it is. For my school band, they provide us with instruments so all the instruments are all passed down. From what my senior told me, there are two Altus's, one of which is in bad condition but we don't know how it's in bad condition. We send instruments for repair only if they really have a problem. (e.g. a note can't be played because xxx key is loose) If I want to get my flute sent for repair, what reason could I state?

Best Answer - Chosen by Asker: 7 months ago
Hey thanks for all your suggestions and compliments! I really appreciate it. I will definitely take note of all your advice.

Regarding my flute, it's an Altus but I don't know which model it is. For my school band, they provide us with instruments so all the instruments are all passed down. From what my senior told me, there are two Altus's, one of which is in bad condition but we don't know how it's in bad condition. We send instruments for repair only if they really have a problem. (e.g. a note can't be played because xxx key is loose) If I want to get my flute sent for repair, what reason could I state?