What kind of grabs are you trying? Easiest of all is an Indy grab (back hand between the bindings)... Actually good jump mechanics should have you lifting your board towards your hand anyway in the air. Work on just reaching for your board to do an indy at first, then try a really quick grab and release, once you're more comfortable and have time in the air you can do an actual longer grab.
Keep in mind that unless you're hucking off of massive jumps/cliffs you'll only be holding a grab for under a second. Probably less than half a second.