Getting a pro rec is the hardest part of the BDCP process. If you get one, you should be pretty much in - given that you don't lie on your application or screw the pooch on PFT or EPSQ. There are many reasons why one may not get a pro rec - here's a few:
1. Navy doesn't need you for certain positions or the timeframe applied for
2. Your package isn't competitive enough (low ASTB scores, low GPA, etc) for the position(s) applied for
3. Your letters of recommendation don't paint the best picture of you
4. "You're name ain't the best in the Navy..."
5. Any combination of the above, or anything else that the selection board may decide and I didn't think of...