I have had that problem before, so I just started pinning my pieces so that they would meet in the end. In other words, if the top is getting pushed past the bottom, then pin the top piece a bit above the bottom piece before starting to sew. Also, try pinning the ends first, then the middle, if there is "bagginess" in one side more than the other put that side down, and they should ease together.
Hope that all makes sense, and you are able to correct your problem. Those sort of frustrations take the fun out of a fun hobby!
Jenna