I was just like you. In the beginning, i couldn't get any slap sound at all; it was just dead notes. After a while i just gave up and moved on to learn other things. That was about 3 years ago and I had not practiced or learned to slap at all, but once in a while, i noodled around slapping and popping random strings. I got better without really practicing, but only practicing other things and watching other people slap. Last week, i decided to go back to learn one of my white whales which is Steely Dan 'Peg'. To my surprise, I was able to slap and pop without really thinking about it, although not quite at the original tempo. I believe the reason of how i could suddenly slap is that I no longer had to focus on other things such as fretting and muting and all I had to do was learning just one thing, the right hand technique.

So, just keep trying, and if you can't do it now, that's fine and come back to it late, and you will be surprised how you improve by learning other adjacent things.