Are you alternating the direction of your throws? Same way each time gives you a granny knot and it slides. Has to be opposite directions each throw to lock as a square knot.

Naledi’s right, that’s the usual culprit. Alternate the throws, and lay each one flat before you snug it down rather than yanking up at an angle. On the thinner pads, less tension is more. You want the knot to sit, not to strangle the tissue. Do twenty in a row tonight focusing only on direction and lay, and the muscle memory takes over from there.