Plastic ball valves are horrible.(I assume plastic and ball valve)
Don't apply the pressure you're hoping to or you'll snap it into fractured parts.
Take it off and replace it with a metal unit witha nylon seal. Better yet, a real gate valve.
For now you can try some food grade silicone spray inside the valve.
Aside:
It's the first I ever saw such a thing but this last winter I closed a ball valve and had water trapped in the ball. I've closed hundreds of them in winter. This one froze, water 2xpanded, and it split the stainless steel ball and the stainless steel housing.
It turned very hard, then sprayed me... Very hard.
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