I got numbing when I first started, I don't think I've noticed any numbing for several years. However, almost every time I drank fresh kava in Vanuatu there was very obvious numbing and increased mucus production in the throat. These experiences were all very effective, so there does seem to be some correlation between strength and numbing/mouth effects. Not perfectly correlated though, because on the rare occasion that the dry kava I drink at home is strong enough to produce a Vanuatu-like kava experience, it still doesn't give me the numbing.
Also, just as some people's body's are naturally more receptive to kava and it's effects...some also seem to be more likely to acquire numbness.