this is most likely to do with skype changing what they send growl, and so something you'd have to take up with them.
however, there are a couple things you can check:
1) in skype's preferences, go to the notifications tab, and ensure that as many of the events have the Display notification option checked. (I can't enable that many for whatever reason. I guess this is what you were after, in which case, I'm afraid you're going to have to go to the skype people.)
2) in the Growl PreferencePane, go to applications tab, select skype, configure, and check that the notifications are enabled in there.
josh
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