I have an mtz processed in P21 and when I load it into the phenix phaser GUI it tells me its P21 (P 1 21 1) and gives me no other options. However, when I run the molecular replacement job it decides that the space group could be either P2 or P21 and runs both - which in my case runs in P2 and gives a partial solution, not the full solution I would expect in P21. A bit annoying as I know my space group is actually P21.
After a few minutes of head scratching, the only way I found to fix it to run in P21 is to enter the "other settings" menu and select "none" within "alternative space groups". I guess this is what your suppose to do, but its not obvious.

Point taken - I'll move this to the main window (and label it more clearly - sorry). The result you're describing sounds like it might be a bug, though; shouldn't Phaser try P21 as well and get the full solution, instead of settling for P2? This is what I would expect for the default behavior.

-Nat