My Pets will not get pregnant even though they are adults.

Yes, they were both in the same household and were my sims 2 dogs.   I did make sure all of the correct settings were in place and the pets did not have any traits that would make them unable to mate and have offspring.  It was extremely weird that when I created their offspring in CAS using the DNA tool, it seemed to have fixed itself.  I hope this helps you pinpoint what could have been wrong.