Issue with restoring site collection

I have deleted a site collection(after taking its back up) by mistake and now trying to restore backup with the same site collection name(so that URL remain same) but it is giving error that-"the site collection can not be restored. If this problem persists, please make sure the content database are available and have sufficient free space".

I don't think this is issue of not sufficient space because when I am trying to restore same backup with other site collection name, it is being done successfully.

What could be the reason? If i am doing it wrong that what is correct way to restore this site collection so that URL remain same?

regards, Ritesh Anand