I just lost my response note, so here is version 2.0, much shorter.
I had a similar problem a year ago where the bucket would only lift to hood height if there was any snowload in it at all, empty it worked fine. Turns out that the hydraulic reservoir plug had spun ever so slightly by the deep heavy snow that was being plowed, causing a slow leak of fluid. Tightened that up, checked all the lines and connections to make sure they were securely fastened, and then all was good.
I have owned my
BX23S for 3 yrs, about 260 hours on it, and that is the extent of my beginner / novice tractor operator experience. Others on the forum migiht correct my thoughts as they would be experts and experienced, so my efforts to answer are just mere guesses. Some comments / questions for you to consider ...
1) my issue was discovered at the height of winter, 2-3 ft of snow, constantly plowing through it, with no clearance at times
2) I had not removed, replaced, adjusted, serviced the hydraulic lines or connections when the problem arose (it was still a young tractor)
3) check that the lines and hydraulic connections (for Loader) are all properly secured and tight so that there are no leaks, and therefore that a hydraulic "loop" provides continuous flow
4) check that there is sufficient hydraulic fluid in the reservoir, and that there is a reading on the dipstick cap, check that the plug below is straight and secured
5) keep the RPM running at a suitable level for the task
6) if it is REALLY COLD / DEEP FREEZE conditions the hydraulics will be challenged anyways, on typical days this is not an issue
All The Best
edit - I knew that I forgot something

7) you may need to bleed the hydraulic lines like you would a brake line to get any air bubbles out (not sure). Perhaps running the tractor and going through the function motions would do this.