Be useful to know what size pots they're in - they should be in 9 inch pots as a minimum, preferably larger. Sounds like they've run out of root room.

Pot bound

thank you both for your answers but I think the problem lies a bit deeper,i have been told that it could be a type of eelworm. the tomatoes are Shirley and are in 10 inch dia pots which I have used successfully for the past 7 or 8 years.

I'm afraid I'm a bit old-fashioned but I would even find a 12" pot a little on the small side. Tomatoes will throw out a new root system from the stem just below the surface when plants are planted a little deep. I would just top the pots up to cover the second lot of roots if you have the space. Shirley has a good disease resistance and I haven't heard of a problem with eelworm especially as you are growing in pots.

thanks to you all