Tried using a bit of counter-weight so you can hang the hold?
Personally I find I use 3 types of grip.
Fully open- using 3 fingers with thumb and little floating.
Semi-open- all 4 fingers, wrist slightly supinated and middle finger a tad flexed.
Full-on crimp- Extended DIPJs and flexed PIPJs.
I assume you mean you can do crimps but can't (physically do the semi-opens. Plus you are too weak for full open?
Anyway, I try to vary the grip when training (the
Moon Routine is pretty good; cheers Buoux 8c).
PlanetScary may also help. Use some counter-weight as required and build up. Try sticking "fingerboard" into the UKB search-o-matic and see what pops up.
Sorry if I'm mis-understanding.
Sorry for the shite pics.