I experienced this twice so far, and think I know what's going on:

If friend sends you battle proposal while you are checking your tasks and you accept it, after a game(s) your tasks will "disappear", but they will still be active, counting your kills or whatsoever.. So in order to see them again, you have to close\reopen worms..

Thanks Towelie - good one! - we've logged this in the bug database ready for the next batch of fixes.