Check the cap on the water tank. Is the clear rubber gasket attached to the tip of the cap?
If not, look in the base of the mop where the tank sits (pictured below) to see if the gasket is stuck in the hole. If the gasket is detached or damaged, you’ll need to order a new cap. You can order one by clicking here: http://www.bissell.com/parts/?sc=part&productid=17074&partid=17385
Make sure to only use BISSELL® formula in the tank. Vinegar and other solutions can cause the gasket, tank, and other parts of the mop to break down.