After you drain the tank and the valve is all the way put a long handled bottle brush in there and run it around the inside opening. Sometimes stuff can get stuck around that thin line where the valve slides. Depends on all what you 'allow' to be flushed down your gray tank. Spraying the pull valve with silicone helps also.
But, every one of mine that started having that problem ultimately needed to have the valve gate replaced. IMHO it seems that's the first symptom before it breaks.
Valve gates are Cheap and easy to replace.