Adjusting hinges on UPVC door?

I have 3 hinges on my UPVC door which is hard to close and lock as it is dropping down at the lock side. I have adjusted the three hinges with an Alllan key. However, a day or two later it is back out of adjustment. Can I lock the adjustment in any way to prevent it happening again?

(PS) The adjustment looks like a cogged wheel with Allan key slot in the middle.

Morag