We have an Aqua Glass Bathtub, with wall surround, that was installed about 20 years ago. When it is used for showering it does not leak. When the tub is used, water leaks around the drain into the garage below. We have had the drain gasket/seal ring replaced twice but it still leaks. My theory is that when a person is sitting with weight centered at the back of the tub, the floor of the tub lifts away from the drain seal. Is this the problem? Anyone have suggestions on how to fix it? The leaking is causing deterioration of the drywall in the garage. Thank goodness this isn't happening with the tub above the kitchen!

Thanks in advance for any help!