I'm making a flemmish string, and find that the twists that splice the loop ends to the string are unwinding when I string the bow and put the string under tension.  I'm sure the twists that splice the loops to the string are in the same direction as the twists in the string.  Years ago I used to make flemmish strings, and don't remember having this problem.  Is it the wax that keeps the string ends from unwinding?  I'm using the same ball of beeswax I used years ago, and while it seems reasonably pliable after being in my hot garage for a while, maybe it has lost something.  I looked at the video from 3 Rivers to refresh my memory, and I think I'm doing it the same as the video, but I must be doing something wrong.

you are twisting the whole string the wrong direction.

Could be you are not putting enough twists in your finished string after you complete the loops. I put a minimum of 15 twists to start with and add more as a string stretches from use.

The other thing you might not be doing is combing the strands straight with your fingers while you are making the second loop. This would be the bulk of the strands between the second loop and the first.

You have to pretwist, reverse direction, the lays between the first and second loop.  Make the first loop, you'll see that this process leaves twist opposite the direction used to create the loop (if you don't let the tag end of the lays "unwind" by themselves).  Take all that out, then reverse it say 15 to 20 turns.  When you twist the second loop, you'll be twisting in the other direction, taking these twists back out.  That's why the call it reverse twist, and the secret to the strenght of any (twisted) cordage.  When you finish, ideally, the center of the string will be straight or nearly straight and the loops tightly reverse twisted (where the twist in lays individually offsets the twist in the lays combined).

Ozy Clint dropped by for a visit while his wife was visiting relatives in San Francisco.  He set me straight.  It turns out that I was twisting the string wrong, without realizing it.  The 3 Rivers video missed a couple of subtle points that I needed to know.  Thanks for all of your comments.