Yes you need full sun, and plenty of space. Most folk who have watermelon
problems have them too crowded. Ther are few insects that bother watermelons
once they get established. The striped cucumber beetle will take down.emerging
plants. Squash vine borers do not bother them. There are a few foliar diseases
that you might encounter. They do need sandy type soil and are difficult to
ripen on clay base soil. To much nitrogen or water can be detrimental to
ripening. But I will bet you are crowding them. Those hills should be a minimum
of 10 feet apart for regular melons and 6-8 ft for icebox melons.
Your lastline intrigues me. The only melons I have ever seen that were orange
inside were orange flesh melons. Red fleshed melons tend to white streaks or
pink white flesh when they fail to ripen.