I have gotten to where I always spray my shellac (HF HVLP gun), unless its on on turned piece, and have none of the problems you described. So, check the following
Wood completely dry. Temperature 70-85F Humidity LOW Out of direct sunlight (I spray outside, but out of direct sunlight) Shellac fresh (best to do your own and the Shellac Shack is an excellent source and make sure you use 100% denatured alcohol)
Shellac has set long enough to cure (two to three weeks if you are going to get aggressive with it)
Deb