One other thing to check is that your walking foot isn't faulty...
When I first started quilting, SITD with a walking foot, I also had terrible frustration from puckers especially on the top layer. Eventually I chucked the walking foot into a drawer and learned to SITD using a hopping foot instead. Many months later I took a proper look at the walking foot, and realised that the 'toes' didn't line up properly - one was bent upwards just enough to interfere with the moving feed-dogs and prevent them from walking correctly. A little bit of elbow power straightened the toe again, and now the walking foot works properly, but I still SITD with the hopping foot instead
K