sounds like not enough momentum.

At the point where you are getting stuck, you should be out of your saddle, shift your weight forward just a hair, and pedaling. You might want to try shifting up into a slightly larger ring to have some power to your pedal stroke.
Between the weight shift and the pedal stroke, that ought to get you over.

You can see in Nanci's photo that she is forward and out of the saddle.

~I.