Watch your wrist positioning. I primarily play a 6 string and I had to work at the slap/pop thing. The trick is (if you play right handed) to get your left wrist around far enough so that your fingering is clean.
The other thing is to try different angles for your right thumb so that you can hit tight for good attack. You just have to practice until you find an angle comfortable for you. Also, the more strings, the less room. Keep your hand motions small but forceful. Takes a little practice, but believe me, if I can do it anyone can.
