That IS very strange... I'm no expert, but almost every Clickbank product I ever promoted had at least 1 sale for about 50 hops. If it didn't, I dumped it.
That's strange. I would think there's something wrong with the referral process. But that wouldn't explain why you are still getting credit for the hops. I would try emailing Clickbank and see if something is wrong. Either that, or that product just converts terribly.