Check to see if the tank valve is leaking. The black, round valve on the bottom should move freely when pressed with your finger. If it’s stuck or pushed up causing the solution to leak, try pressing it to come back down.
Additionally, make sure there are no cracks or holes in the tank. Replacing the tank will take care of your issue in either case. Here is a link to the tank: http://www.bissell.com/parts/?sc=part&partid=26893&cid=ccsocial