I find if I use my phone to pay using Samsung pay (Samsung S9+) then my galaxy watch 3 doesn't work for the next purchase.
So I reset my watch which then does function correctly until I encounter a paying device that doesn't recognise my watch payment so I use my phone and then I am back to my watch not working using Samsung pay?
Had a similar issue with Active 2 watch linked to s10+. After talking to help desk a few times it was recommended to remove the card being used from phone and watch. Then re-add and pair with watch. Touch wood worked OK since.