How much heat? Could be too much. Or it could just be random poor germination. Not everything germinates, after all. I planted two or three watermelon seeds and only one germinated, 2+ weeks after it was planted -- and it had a false start, too (its first set of true leaves formed wrong). But it's a nice, bushy, beautiful plant now.

My Ronde de Nice squash took 3-4 weeks to germinate. You never know.

Possibly putting them in a sunny window area. Watermelons are heat loving and normally get heat directly from overhead with ground somewhat cooler beneath the surface.
Older seeds ususally lose ability to germinate. Possibly that is a problem.
I usually plant 2-3 times more plants than I need. Then if some don't come up, I'm still covered. Some may also die of disease or insects.