But (there's always one, isn't it?), you said your other plants are thriving. Is everything planted in the same type of soil?
Stick your hand down into the soil and see if it's moist. About as simple of a moisture meter as you can get!
I see that you've got small fruit on a plant or two. There are SBV (squash vine borer) that can cause something like you see...but, the leaves and stems will start collapsing because the borer's have blocked off water uptake in the stems. You're look mostly healthy, though. I do see a tinge of yellow on a couple of leaves but nothing overwhelming.
Stick your hands in there.
