More than likely, your seal has crud stuck to it. You should keep the slide valves and rubber seals wet. Keep a gallon or two in the tanks so the water will fill around the neck of the outlet. In winter, drain all water and flush all contents .... all,,, then pour some TV antifreeze down. This keeps them wet over the winter.
If they are leaking, try some valve slide lubricant sold at Wall Mart or your local RV dealer. Put some in the toilet or down the sink. Then work the slide valves back and forth.
Other possible cause of this does not work, the casing around the valve could be cracked.