Just a matter of waiting really. Does the greenhouse get good sunshine?

if they are taking too long too ripen pick them put them in a dish and put them on our windowsill in doors and within a week or two they will ripen.
I have done this many times

Not sure it does much for the flavour though and they are hardly fresh if left sitting for two weeks.

AND ive not had no problems with my tomatoes doing it this way and they are as fresh as when i put them there