Can't do aimed shot against certain mobs?

I running through the vault in Preoria and have entered the first heavily radiated (green) room. There are a couple of roaches and a Roachor. When firing at the roachor, aimed shot doesn't seem to work.

I'm playing in TB mode, no fast shot and I have selected aimed shot, but the interface for choosing body parts doesn't appear when targeting the roachor. Instead, the character shoots a single un-aimed shot that consumes as many APs as an aimed shot. So far this doesn't appear to happen with any other mobs. Haven't found anything like it on other forums either.

Anyone here know what might be wrong?

Don't do a reinstall, this game is just heavily bugged, and there are areas of play that just don't make sense bug wise.

However the action you describe seems like a single shot with a pre-defined target area (right click the yellow shot icon on single shot card). You can pre-define the target (I typically set head).

If all else fails... its just a bug.