Sounds like the problem is valve in the tub spigot is bad or has something blocking its movement. The only thing that valve in the tub spigot does is block water from going out into the tub so the water goes up to the shower instead... at least that's true for 'sticks and bricks' tub/shower faucets.
I don't know how you get the tub spigot off to inspect, clean, or replace it. Hopefully someone here knows.
Sherm & Terry w/rescue Eydie (min Schnauzer) & Charley (std Poodle)
SOLD:2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18, 365+ '20
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