Positive OPK one month, the next nothing, still ovulating?

Jessica • 29 y.o living in NY married to my amazing hubby on 6/7/14.
Last month I had a positive (smiley face) OPK..this month, did the same routine, and nothing, however I know my body ovulated..has this ever happened to anyone? One month yes the next nothing but still ovulated? Care to explain why this may have happened..and can you still ovulate even if you didn't get a positive one?