A good teacher (who knows how to slap) will be able to observe your bass and technique and set you on the right path. In-person would be best but via Skype could work too. As far as the actual sound is concerned, there’s no reason that, given the right bass and set-up, you shouldn’t be able to produce a good slap sound. It could be your technique but it could also be the bass, the set-up, the strings, etc.

Even a guitarist buddy of mine could pick up my bass and slap the
out of it, but not me.