It would help us greatly if you'd tell us where you live.
Also how are you taking care of the plants? -- water? fertilizer? in pots or the ground?

For starters, if the prevailing temps (day or night or both) are below 55F, it's too cold for tomatoes.

I live in Central Florida so the temperatures aren't too terribly crazy. Every now and then the temperature will drop to 40 or 50, but that doesn't happen very often and doesn't last very long when it does happen. It shouldn't happen any more as we are entering spring and summer.
I normally let my sprinkler water them which goes off every other day, unless it is so hot outside that the soil dries up and I have to water them.
I don't believe in synthetic fertilizer and I don't use any compost or manure so thats a no on fertilizer.
Yes, they are in the ground.

1. GOOD TRANSPLANTS ARE NOT BIG....LOOK FOR TRANSPLANTS AROUND 6 INCHES OR SO
2. TOMATOES NEED NUTRIENTS TO PERFORM WELL...GOOD SOIL PREVENTS ALL KIND OF PROBLEMS AND DISEASES WHEN TRYING TO GROW TOMATOES (OR OTHER VEGETABLES)...SOIL NEEDS TO BE LOOSE, CAN RUN YOUR HAND DOWN INTO IT...IT TAKES TIME TO BUILD GOOD SOIL...
I DON'T KNOW HOW TO BUILD GOOD SOIL WITHOUT ADDING COMPOST FIRST AND FOR TOMATOES MYCORRHIZA SECOND...JUST TRYING TO HELP...ROY