I have seen the top slide to one side of the bimini frame tubes making it very difficult to get one side of the curtains on. Also, the front and rear guy-straps will move and I have seen broken buckles and snaps from pulling too hard to get them on.
Try removing both side curtains and make sure the top is centered on the frame tubes. Install the front panels first, adjust your straps correctly, and then the sides should go on.
Warming up the plastic windows will also help a lot. And you can pull the top over with one hand and put the snap on with the other. They can be really tight! But if everything is lined up and in their correct position they should go on.
EDIT: I replaced my adjust buckles with SS ones, the plastic ones get brittle from UV exposure and will surely break in an inopportune moment! The new buckles are easily hand-sewn in. I've been in a couple of storms that I thought were going to rip the top right off. They are really strong!