Too many reasons to count... maybe you can do a google search and compare what your plants look like to some of the images you find on google and go from there? Good luck!
You know it! But, they won't want vegetables to eat - they'll want TACOS and BROWNIES! LOL
Okay, here's some tomato advice:
Try dissolving some epsom salts in water and giving the tomato plants a good watering.... I don't think it can hurt and sometimes it really , really does help.
Are they in the greenhouse or ground? If greenhouse, possible over or under watering. If in the ground, the same, plus possible bug pressure or disease.