Any chance one of the girls is actually a boy? Or is the weanling tub comprised of just the 11 heavily pregnant young does?

I can only speak from my experience, but have never had a 4 week old buck able to do the job. Your line is very virile

If it helps, just imagine that if you really wanted a group of does bred and put in a 4 week old buck, you wouldn't see a litter for months. Murphy's Law
(I know, I know - that doesn't help. I tried!)