Interesting. My tractor has the opposite problem when it is cold out. In order to raise the 3 point you have to lift the bucket up and hold the 3 Point lever in "UP" while lowering the bucket.
First things first, is the tank topped off properly? (probably, but had to ask...)
Second thing I have to ask is the suction screen in the tank (or spin on filter) clean?
If your system is plumbed the way I think it is, the pump feeds the loader valves, then it goes to the diverter valve for steering then back to the 3 point valve. There are two relief valves in this system. One in the loader valve and another in the 3 point valve. The one in the 3 point valve is for when the loader is disconnected or if your tractor never had a loader. This is the main pressure controlling valve for the entire system. You can never get more pressure than it will allow.
The relief valve in the loader control only affects the pressure available to the loader cylinders. In theory it should be set the same as or slightly lower than the main relief. A piece of dirt in the seat of either relief valve will not let them reach full pressure. Also it is not all the unusual for the relief valve in the loader control to come "loose" and lose pressure. As in the adjuster unscrews slightly, lowering the setting. It is constantly being pressed on by a spring and I have never heard of one "screwing itself in".
That's the basics. If it were my tractor the first thing I would do is mount a pressure gauge to a quick coupler and plug it into the upper (pressure) auxiliary port at the back of the tractor. When you pull the 3 point lever to full "up" it will display the maximum pressure allowed at the 3 point cylinder. I would think that you should be seeing at least 2300 PSI.
If you have good pressure there, then the problem is elsewhere. Likely the loader control. Probably the loader pressure valve has backed off some. Or it could be a problem in the valve itself. If you have good pressure to curl a heavily loaded bucket or something on forks but not lift the bucket then it is likely internal to the valve and not the pressure relief. That probably means take it apart, clean and inspect in order to figure out what is going on.
Please ask more questions if you have them and by all means let us know what the final solution is so others may benefit from your post.