check the webbing
I had a similar issue with my homemade toggles. I determined that my webbing was bunching up as I created my marlin spike hitch where the webbing wraps around the toggle. That was creating a much smaller surface area that was contacting the toggle, so when weight was applied, the knot cinched down tighter on the toggle with a small surface area, thus contributing to a collapsing toggle. It didn't actually fail, mind you, it was just an observation that I made so I discarded the toggles. BTW, I weigh about 250.