When this has happened to me, it turned out to be poorly designed. Deb is correct the denser the design the heavier the stabilizer should be.
The more stitches there are the more stabilizer you need. The problem may be the designs, or your thread. Are you using embroidery bobbin thread? It is finer than the embroidery thread for the top. The only exception to this is for free standing lace. If it's the designs themselves, did you have to convert them? I've had a few not convert well. You can contact the digitizer for help. Sorry, all I gave you was more questions.