Don't know about how anyone else learns to play, but I have to practice some things more than just a few times.
Part of learning slack key for me is collecting turn-arounds. I figure I just can't have enough of them in my bag of tricks. So as I was listening to Sonny Chillingworth (for the thousandth time), I heard a turn-around I didn't have.
It took me the better part of an hour just to get the fingering I wanted. This one had a strange fingering for me so it took a bit to make it sound good. Once I was comfortable with the fingering, sound, etc. I went on to work on timing, modulation, speed, etc. In all, by the time I was satisfied I could play it how I wanted it to sound, and had it to where I could comfortably integrate it into my playing, I estimated I played this 2-3 bars over 800 times in the course of a day (two sessions). I'll probably still have to play it several hundred more times to be really, really comfortable with it.
Of course, I wish it were easier - maybe others pick things up quicker. But this seems to what I need to do to learn to play some things.