We had the same problem from petco. We bought a what we thought was a breeding pair from petco but 2 months later nothing happened so we feed them to our bp. We found another pet store that sold mice and we found that we bought a very large healthy male that was full grown an a large healthly female and we have had 2 litters from them so for and our colony is getting big. So we found that we figured not to go with the small ones cause they could be to young so we switched and it worked. And also filling up half the tank with pine bedding as well. to make them feel like they can dig a den and mate and have babies that also helpped int he matting process.