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The slot where the valve blade travels may have debris in it, like toilet paper and won't allow it to close. Usually you can clear it by having some water running in the toilet via a hose while you push the valve back and forth. If it won't clear you may need to take it out and clean it. IF this is a cable operated valve, the cable may be out of adjustment. Those are the usual things that cause this to happen. Hope this proves helpful here. Please get back to me if you need more later as all follow ups are included in our fee.
If you are having freezing weather it might be some ice buildup in the valve.........kind of rare, but it can happen.
Just checking back to see if you need more input from me on this issue or did you get it resolved?