Magic mouse bluetooth problem

I have a blutooth dongle on my mac. As batteries were getting low in my Magic Mouse I removed the old batteries and put in new charged ones, something I have done numerous times before without a problem. On this occasion I cannot get the mouse to pair with my computer for some reason. I have checked using Bluetooth Explorer and the dongle is shown as active. (The Blutooth provided in the Mac is useless hence the dongle) Any suggestions?