While you're having your cloth re-stretched, be sure to have your slate re-adjusted, and have that "spackle" replaced with some bondo.
Sounds like you need to enlist the help of a qualified table mechanic.
I was having the same issue with the seams. When we went to fix those it turned out the cloth was super loose and the rail bolts were not even tightened. We redid the seams , stretched the simmonis tighter and properly tightened the rail bolts . Table plays great.