What type of lube are you using?
If you are using a spray type lube, Hornady, Dillion, etc. You need to let the carrier liquid evoprate off the case prior to sizing it. Otehr wise it will get stuck. Doesn't make sense I know but if the lube is still in the carrier liquid it will get stuck.
I am curious if the case was harder than normal to size? If so why didn't you stop and lower the ram and check the cases progress?