If your plane is about 2.3-2.5 kilos (or even some more too) then your engine should have no problems climbing of of a hover with the 12.25-3.75 prop, and most every other prop to. Is your engine tuned properly? what rpm`s are you getting?? A healthy 50 size engine should manage to pull 4-4.5 kilos and will be more than enough to pull out of hover.

My own favourite synthetic prop for 3-D is the APC 12-4, or I use wooden 12-4 and 13-4`s on my 50 size 3-D engines