I'm trying right now to learn Higher Ground, by the Chille Peppers, as my first venture into slap and pop. I'm having a problem though. When I slap and open string, it works just fine, but when I try to slap a fretted string it just clicks against the fingerboard and doesn't tone. If anyone has some helpful advice on slap and pop I'd love to hear it.

Get some slap teaching videos, and see how their doing it. Fleas got one

I wouldn't recommend the flea video, since he is slapping vertically, and IMO it is better to slap horizontal

im going to explain slap as it was explained to me...

no one can teach you how you are going to slap. you can watch people do it, you can have someone show you how they do it, you can tear apart the theory and you still wont know how to do it....you keep on trying different things until you get it right.

i got it in pieces like you seem to be, i nailed the pounding (thumb) part, the i got the hammer-on + slap, etc...im still learning new tricks and approaches still

what im getting at is you have to find your touch on your own.
thats my $0.02

I have the same problem, but im playing a fretless, so im guessing its that:O I hope so atleast.