Post a picture of one of these animals.

If you've tracked it to a bull and you're convinced it is the issue, I'd get it over with and cull his daughters. I'm finishing up doing something similar with a bull I had ~4 years go. Every single daughter was eventually a failure. It just is what it is, get them out of the system. 

If you're not convinced, I'd look into the health side. It's possible it could be that bull, but not his genetics at all and he brought you something even more fun to deal with. Have you considered testing for Anaplas? Have you ever had an outbreak of Red Water? It's possible for both of these, and many other things, to fly under the radar and wreck young cows without causing the death loss everyone associates with them. I'd pull blood on a couple of the affected animals and have the vet run a full panel of tests on them just to eliminate several potential causes.