Dave, Only through inspection can you determine the answer.
Are you on the hard? Then you have more options. You can inspect from underneath and determine if something has clogged the valve causing the "Sticking" issue. If you are in the water, then you have fewer options. Safety says plug the thru hull from outside before using a "Lot of force" to fix the "stuck" ball valve.
My experience.
This valve worked smoothly from the time I bought the boat to the day I felt I needed to change it. No reason, just a feeling it was time to change. I had a bit of "unexplained water" in the bilge area where these valves were. Just a sponge soak up amount maybe every 45 days.

Looks aged and perhaps the handle could be changed, but the handle worked and the valve worked.
Till it didn't.
When I decided to pull the boat and replace the valve. With the boat hanging in the sling, I was inside the boat. I reached down and started to turn the valve (unscrew it from the thru hull). It broke apart into two pieces.
The pink metal tells the tail of zinc corrosion from the metal valve.
It broke into my hand, not wrench needed. I was chocked and pleased I had decided to do the replacement.
If your issue was mine, I would get the boat out of the water. Investigate the thru hulls and valves. Remove the valves from the thru hulls and either clean them up and free the working valves or replace.
If the thru hulls look pink or turn when trying to remove the valve, then remove and replace.
You have nothing to loose by making sure your thru hulls and valves are in good working order.
You have your boat to loose if they are not.