Watermelons in GrowBags

I have been growing watermelons for years. When I first started, we had a very large in-ground garden and my watermelons flourished year after year. I had so many every season that I would give them away. We have since moved and I have to use grow bags on the side yard due to too much shade in the back. My tomatoes and peppers thrive in the grow bags, but my watermelons are pathetic. They'll grow a single small vine and produce one watermelon that gets about the size of a softball, and that's all she wrote for the season. What am I doing wrong? I'm in midwest Georgia (US), they get 8+ hours a day of sun, and I use Fox Farm soil as everything I have planted in it does very well. Bag size- 15 gal, mulched with wheat straw on top