I cant say what the problem is off the top of my head but. Though I do know that tomatoes usually require added calcium, and that a lack of enough calcium can look like many other problems so it is hard to distinguish that as a problem. But here are a few links about
nutrient Deficiencies and Toxicity that may help you.
Nutrition (this is probably the best help for hydroponic tomato's)
Nutrient Functions and Deficiency Symptoms
Informative Articles - Common Problems
Symptoms of Deficiencies and Toxicities - Greentrees Hydroponics
Nutrient Concentration and Function in Plants