Were the scrapers filled and used to compact the bottom while not scraping/moving dirt? Scrapers will work, if used that way by a person who is familiar with that technique.
Do the polymer/bentonite test again, this time using a tube that is as tall as your pond is deep. Water exerts approximately 0.5 psi for every foot of water depth.
What seals in a 6" to 8" cup might not seal on the bottom of a 15' deep pond................
