I think you are playing an open snare between your tap and your hammerons. Well, it's not incorrect, but I know what you mean. With the normal tapping you should allready lay your finger on the position you want, so if you want to tap the 12th snare and hammer on from 5 to 8 (-t12-5h8-) basicly your index finger of your frettinghand should allready lay on the 5th fret. Don't tap, then lay your finger on the wanted postion, but lay you're finger in the wanted position and TAP then hammer on. Hope it helped.

Latz, Gulder