A good read...
http://www.petshrimp.com/articles/whyshrimpdead.html
I had the same problem a while back and attributed it to two things. I had gotten a little lazy on the water changes and I was overfeeding. Now it didn't seem like it and at first they would rush out to gobble it up, then less enthusiasm, then I lost a berried shrimp, then another shrimp. I think perhaps the berried females are under more stress and this could explain why the are affected first?
Back to the old schedule of weekly WC and feeding every other day and no more problems.... Perhaps not your trouble, but thought I'd share a similar experience. The key to seeing a potential problem is lazy or colorless shrimp, this means trouble in the water parameters somewhere. The little guys should be constantly picking aroung the tank when happy and healthy.
Good luck, Bill