Frost Me,
Because you are back-knotting, there are actually 4 threads for those first few pearls.
Which threads are you pulling apart, to seat the knot and tighten it?
I pull apart the 2 threads that are attached to the needle. They tighten around the original 2 threads that you strung the pearls on before adding the clasp.
I mention this because I noticed on Lloyd's video (watch from 6:27 to 6:45 and from 7:30 to 7:40) he does it differently.
He holds the 2 original threads (on which the first 3 pearls were strung at the start) in his left hand and the 2 threads coming from the needle in his right hand, and pulls
those apart. That action moves the knot down toward the first pearl. I grabbed a screen shot at 6:36 so you could see this.
I do it a bit differently. After forming the loose knot between the first 2 pearls, I guide it down toward the first pearl with an awl (but you could also slide the knot down just by pulling the needle threads apart.) I then take the two threads that are in his right hand, which are attached to the needle, and pull
those apart so they tighten around the two original threads that are in his left hand-- as in my photo above. That makes a nice tight knot for me.
I'm not saying his way is wrong and mine is right, just that my way works better for me. I suppose you could use a combination of the two ways-- slide the knot into place as Lloyd does, and then add the step of pulling the two needle threads apart as I show in the photo.
If your knots still don't hold well for you, then you might want to add a dab of glue. Serafil and Beader's Secret are slippery and I can see where knots might come undone more easily than they would if made with Power Pro or silk.