What causes tomatoes not to ripen?

Healthy plants tend to over blossom. If the tomatoes don't get exposed to enough sunlight, they will be slow to ripen. You can try spreading the plants or even breaking some of the non-flowering branches to expose them to sunlight.

lack of water, or too much water, too much fertilizer, planted too late,

lack of sunlight, too hot. If the weather has been extremely hot they are late in ripening

Try to cover your tomatoes with a light weight veggie gardencover sold at most gardening centers Tomatoes don't likeextreme temperatures.

If temperatures are cooler than usual, tomatoes may fail to ripen. Also there are a few varieties that remain green even when ripe, although you would probably know if you had them as they are rare. Another reason could be that it is just a variety that is large or slow to ripen.