Access to com port is denied error..

I am working on inventory control project using RFID. I have connected my RFID reader to com port 12 of my laptop.. But when i run my software first time it works fine, i can read tag ID properly. But if i close this software and reexecute or if i switch from one window form to another then i am getting this error on the screen - "access to com port 12 is denied" And when i get this error, my c# software is not reading RFID tag ID properly.. In short 1st time it works fine but next time i am getting this error..please help me.