Not sure i can be of much help, but it maybe the compost you are using, lack of food, to much or too little water or the temperatures could be too hot during the day and to cold (especially towards the end of summer) during the night. I pressume your only growing greenhouse varieties and that you are training them according to whether they are cordon or bush varieties?

Many thanks for your reply it's most appreciated, however for more years than I care to remember I never had any problems and then for some reason at least 6 years ago it all went wrong.I have dug all of the compost out of the green house,tried growing them in rings,growbags,direct into the boarder all sorts of varieties,but always the plants start off well and its only after that the tomatoes are nearly ready to ripen that it is noticeable.
They have no shine to them and have to be chucked away.