If you have the same type of valve - the standard sort of Guillotine - with the blade held between a rubber seal - you might find gunk at the bottom of the seal that can be cleared out with something like a coat hanger type wire. It is not a fun job. I fixed ours once just digging gunk out of there. Some people take the valve out to fix them. A lot of cleaning and rinsing and then if you can jack that side up to get anything left in the tank to slosh away from the outlet. 4 bolts/nuts take the valve out. Be careful to sort of pivot the valve out and insert something between the pipes to keep the slot open for when you put the valve back in. You might even try some rubber lube on the seals. We will be down that way but not for another month so not much help here other than suggestions.