Pat,
I place the string on a hook in the garage, and pull hard on it, then hang about 30 pounds on it a while.
This string has 200-300 arrows down range already.
My suspicion is that the knot is slipping, like you say.
I have a twist or less per inch in the string.
I may have to make it initially with a higher brace height, an inch or more, then shoot it in, instead of chasing the perfect height right off the bat. OR, trade it, as the B55 did not slip / stretch.