Tomato Problem

I have a problem with my tomato plants.  They are growing well but the centre shoot and off shoots are tightly closed like a fist shape,  I have checked the roots and they appear to be healthy and I have checked for green fly but there is none.  They are planted in soil mixed with cow manure in a large well ventilated greenhouse.  Has anyone else had this problem or could enlighten me as to what my problem might be.