Sounds very possible. Could also be some slight blockage in the magazine tube. If you pump gently the shell is not pushed (by the weak magazine spring) past the blockage, but if you pump vigorously, the act of pumping helps to "jump" the next shell past the blockage.
How's that for some long-distance diagnostics?
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.