The reason we're being so persistant is that every one of us can:

nose our plane down, land, take off,

nose our plane down, land, take off,

nose our plane down, land, take off, and keep doing it all day long.

So we are as perplexed as you are as to why it is happening.

If we can do it and you can't, you MUST have a problem somewhere.

If there is nothing physically wrong, then there's only one answer to your problem - You're flying too long and running too low on fuel.