I had watermelon planted in the same place last year. The vines grew like crazy but I never got any fruit. The potatoes never even sprouted, they were eaten away before they had the chance. Prior to the watermelon nothing had been planted there for several years. This is a raised planter.
A soil test could clarify the mystery. You may be lacking in P or K or some micro nutrient.
Good to see you on this forum - do you have any friends and/or neighbors that garden?
Sharing trial-and-error with others helps everyone learn - I've gotten (and given) some pretty good pointers from others in my zone - and it's great fun, too!