Help! I must be doing something wrong.

Dear Spray-n-grow:

I must be doing something wrong.  I have been spraying my vegetable garden (tomato, canteloupe, and watermelon plants) with Coco-wet, Bill's fertilizer, and Spray-n- grow every two weeks.  My plants look really good until I spray them and then they turn yellow and some have even died. It normally takes about a week with out Bill's mixture for the plants that lived to look better. What could I possibly be doing wrong? 

Can I root feed instead of foliar feed?  It is super duper humid here so I wasn't sure if that was making a difference too?

Please help!

Tiffani Stinson