What are they in?
An inch or two of potting soil?
I grow watermelons fine... But I'm in the sand.
If you are in the heavy clay, I'd dig down into the clay at least shovel deep, and work in lots of compost / manure, and try some new seed. Should be plenty of time to get some watermelon, before frost.
Incidentally, watermelons require all day direct sun... If those are in the shade, that could be part of the problem.
