EVA Problem- can't engage grappling hook

I've built a shuttle, using a cargo bay as the crew compartment. I have my two astronauts sitting in rocket chairs. In space, I open the cargo doors, exposing the astronauts. I highlight a chair and click on EVA. The astronaut lifts out of the chair and I press C for 1st person view. I place the little cross hair on a part of the cargo bay. When I press my mouse scroll wheel (serves as middle button) the grappling hook should engage put it doesn't. Then the astronaut moves away and I lose him. First, am I activating the astronaut correctly? Second, why is my grappling hook not engaging. My mouse was knocked off my desk by one of my cats several days ago. Could that be factor. The mouse works fine otherwise.