I've had similar issues, where getting melee killshots resulted in no light for several mobs in a row, which was a problem since we needed pearl for increased chests. But after a couple minutes we started getting pearl lights every melee kill for a while, then it stopped for a minute or two again, and so on.
I'm starting to beleive in the "if there's too many chests up in the zone, you don't get light on killshot till there's slots available" theory.
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