Looting 3 bug on withers skellys

I've been camping like crazy in the nether trying to collect up wither skulls to beat the wither for a beacon. the first skull i got was on my first wither skelly kill with Looting 1. since then ive killed maybe 500 wither skellys with no skull drops and i have upgraded to Looting 3. i don't know if this bug is an enchanting/Looting bug or if it is to do with the skull drop rates but at this rate ill be spending the next 6 months (IRL time) trying to get a beacon.

Please look into this issue.