I actually have a very similar situation going on as you with the opposite results. I had a dozen blue velvets in my 15 gallon that they grew up in. As they came into breeding age (only 2 weeks ago) i moved 8 of them into a deep blue 3b model, approx. 2.5 gallons and since i have 2 females berried, and the other 3 with saddles that are ready to berry any day. Params are identical as the 15. What all do you have in the 2.5g tanks? I have a lot of moss in mine which i did to help relieve stress in such a small tank and increase surface area for the shrimp to be able to move, hide, etc given the small dimensions. Can be difficult to find the shrimp in it all, but so far so good on the breeding end, i just get my updates on pregnancies during feeding time. Just my on going experience, we'll see where it goes!