I’ve experienced my borosilicate glass plate teetering quite a bit recently.
The first time, the stock heating bed was convex, so even when I put a glass plate on top, it teetered.
If you know your heating bed is not convex, then here is what I’d try:
If you have a silicon thermal pad between the glass and the heating pad, try removing it.
I’ve had some pretty big bubbles or lumps between the glass and the thermal pad that need to be “ironed” out. Removing the thermal pad takes that possibility out of the equation.
Try tightening the leveling washers as much as possible to “reset” the level of the heating bed. Then try putting the glass plate on top again. Does it still teeter?
If not, try leveling again.
If these things don’t work, then you may have a more serious problem with your heating bed. I’ve had some pretty good conversations with support at monoprice, and can recommend them too.