There are a few reasons why a guppy or any live bearer may have breeding problems... The few I've noticed is that sometimes they wont feel safe letting there fry out... due to other fish, or not enough hiding places, or water problems.. But all the above are basicly due to the mothers refusal to give birth.... Other problems can be like in any other pregnancy...

Like maybe she has too many fry inside of her which makes it harder for her to push them out. (not having the muscle to contract due to it being so streched. Or other problems can be from past injurys, or blockage.. I noticed on mine that it had trouble with the first bunch, but as she continued it seemed to get easier and easier for her. enough that just before birth she would squirm around rather quickly before pop, a baby pops out. later she would just sit there, and pop...

Id try giving her a less stressful surroundings, maybe something to hide in, or to give her fry somewhere to hide in, (I had to save everyone from the jaws of death named mother. well almost everyone, she's fast)

Dont know if i've helped any... If i did, yay :) if not, well i tried :) Good luck, maybe it will just take some patiance on both your parts..