I recently encountered a 'bad' valve core. This valve core worked fine in its original tube. It just doesn't work once I put a valve extender on there. The issue was that the tapered rubber ring was too big and loose. So when I tried to pump with the air compressor, it swelled (swole?) up and blocked the air. I am guessing the extender has a slightly smaller ID than the original tube valve.
I replaced it out with a Continental valve core and all was fine. The Continental valve core has a noticeably tighter fitting rubber sealing ring.