Transplant shock, too cold, too dry, not enough fertilizer. Any of those sound like possibilities?

How warm is it? That's what determines how much they could potentially be growing. Unless the soil is above 60F and preferably 70F and ave air temperature the same, they won't grow much.

Don't know what zone your in but I have never grown anything here in zone6 PA in the amount of time it says on seed packs, Don't know were they get some of their information from.

Transplanting melons sets them back. The dates assume you've sowed the seeds directly outside.

9 out of 10 is that it didn't get warm enough
in your area. When it does they will take off!