I've had this happen as well. It always happens when the situation is very hectic and I am switching targets quickly. However, I believe that I am really at fault here. I think that the audio report (e.g, target acquired) is just too slow to keep pace with what is actually occurring. So I click the button to lock, hear nothing, and assume that my reticle was not in place, so I click again and hear the unlock message......
I'd like to see WG add an option to select tones to indicate the status of auto-lock instead of the lengthy messages, and the system could be more responsive.