There are a hundred reasons, here are some of the most common:
1. "She" is actually a "he". - Yes, two males will still lock up on occasion.
2. Stress. Either her, him, or both.
3. She/he isn't mature enough to actually become gravid/get the job done. Age and size play equally important roles in breeding for both sexes.
4. They were not cooled long enough or at the right time. - She may have been "about ready" and you raised the temps on her. Or she may have been ready just before you cooled, but you missed out by a week, a month, a day.
5. The first few lockups may have been "dry runs" - it can take multiple lockups before anything happens and even then nothing is guaranteed.
6. She may be gravid and you just missed the signs. If nothing happens within 2-3 months after the last time they were together, then you can most likely be sure she isn't gravid. Otherwise, you never know... they like to play with our patience and our minds.