Time and patience, the weather turned cooler so they are taking longer to ripen than they would if it was hot and sunny.

There are some tomatoes like Green Zebra and Green Grape that NEVER go red. Grow some plants and give them to friends, just to infuriate and confuse them! (The flavour is actually very good)
Seriously though MG is right and the large tomatoes always take ages. Once one starts to turn, they all do, so soon you will find you have a glut.
If you are really impatient, you can always harvest one as soon as it turns pink and put it in a plastic bag with a ripe banana. The ethylene vapours from the banana speed the ripening process.

the variety makes a difference as well some are classed as early -- one i've grown this year called 'stupice' cropped on 4th july! and is still going strong