Trim keeps changing once I get airborne - XBox X

I am a newb on FS for XBox X. When I take off I often find the plane wants to descend.
So I try to adjust the trim. For a start the trim is always moving and then if I select and try to adjust, it makes huge changes for just a slight R joystick moves. But then it goes back to moving so it’s pretty pointless! I set the trim to +1 before taking off. So why doesn’t it stick at 1?
Help me please,
Tks
Nigel

It could be your Assistance Options.
Ensure you don’t have any piloting assists on.

Thanks, I don’t think it is but I will check.

Which aircraft?

I have used a few light singles, twins and the same thing happens.