When you killed the werebeast, did you get a text box that confirmed you completing the quest? You can slay dozens of werethings, but if you don't get the single one they want then it won't register as complete.
If you
did get a confirmation text box then the quest could be bugged. This has happened to me before.
Alternatively, you could've had the quest where the "werebeast" is actually a bear or some other animal. If this is the case, you should still get a text box confirming quest completion.