I am experiencing the same problem when I use Mug without either of the kisses turned on or with just Kiss of the Wasp turned on. In other-words I was getting mug procs for the first week or so, but ever since I have gotten absolutely no Mug Procs whatsoever.

That said the Mug ability appears to be working properly when I have Kiss of the Viper turned on, as such I have taken to just using Kiss of the Viper as the self-heal is actually very good when soloing, and even in dungeons on occasion.