Please help. My '02 Yamaha C90 is slowly creeping down. Power tilt and trim work fine, I checked and looks like plenty of fluid up to the reservoir screw and no noticeable leaks. It is my first boat for 2 years now and I have never had this issue before. I tilt the engine up off the ground about 4-6 inches and it is on the ground within an hour. I have double-checked the manual tilt screw and it is tight. I am trying to scour the internet to find out what to do and getting nowhere. Something said something about checking a check valve but can't find it/I don't know where that is on the parts diagram (below).

For convenience here is the parts diagram (not an endorsement for the website)
POWER TRIM TILT ASSY - 2002 Outboard 90hp 90TLRA | Crowley Marine

Any and all help would be greatly appreciated.