Breaker box below electric meter
Sometimes panel manufacturers will have a null breaker slot on top, bottom, and/or both. Even though a double breaker seemingly fits right in like it's supposed to, only one phase will be hot.  

A visual inspection of the busbar is one way to tell, or if you have the busbar layout diagram it will show too.

In case this is the problem, simply move the double pole breaker into the proper slot, then put a blank slot filler to cover the exposed parts.  If you don't have a blank handy a breaker with nothing on it is fine too.