How come my time capsule does not go past the "preparing backup" phase?

I have two other laptops that seem to be backing up to my time capsule without any problem. However, I have one laptop that seems to be stuck in the "Preparing Backup" phase. I have restarted the time capsule and the laptop, but I have not had any success in backing up the data on that one computer. It was working fine before, but it seemed to stop from April 29, which is when the last backup occurred. Can anyone help?