can you still twist turn it a bit or totally stuck? happened to me once but it came off after wriggling it for awhile. for my case, it happens if i don't align the marking on the tamron cap to the orange dot on the lens, since the rear cap could fit the mount at several places.
Tamron and Sigma lens cap very annoying, only can put on when in a certain alignment. If you put it on in another alignment without realizing, it will get stuck like what happened to you.
I just bought some Pentax replica ones from eBay. They don't work as smoothly as the original Pentax ones but for the price I ain't complaining. Mine do have the "Pentax" "Japan" wordings, but if you find one that looks like the Pentax one but doesn't have the words, you might have better functionality. The replica I got from FW works exactly like the original Pentax one.