OHKO moves are very particular. There are two main reasons they do not work:
OHKO moves have extremely low accuracy. It has just a 30 hit / miss rate, which means it's very unlikley with the low amount of PP each move gets that it would actually hit at all.
OHKO moves only work if you are the same or a higher level than you're opponent. If you are at any lower level, then these moves will not take affect.
Note that the accuracy does actually increase for each level higher you are than you're opponent by around 1% a level.
Source: this link and own experience
