First of all, do you have any ventilation in your greenhouse? Your tomato plants may be getting to hot.
How big are the pots that the tomatoes in? They need, at the very least, a 5 gallon pot.
If you are growing your tomatoes in a greenhouse, you need to check them for bugs.
White fly and spider mites LOVE tomat
oes. You'll see the white flies if you disturb the plant by running your hand through it. Insectidal Soap will get rid of them, as long as you are sure to spray UNDER the leaves, where the eggs are.
You should also find a good magnifying glass and look under the leaves for spider mites. They look like tiny red dots, and will move if you keep your eye on them. They are extremely tiny, but multipy by about 30,000 per minute, and the mites not only suck the juices out of the plant they are on, but the they also quickly spread to surrounding plants. There are sprays you can use, but the best method is to throw the infected plant into the garbage and burn it.
I wish I had taken pictures when I had the greenhouse...If I had, I could show you just what they look like.