Hi sbphinney, When tomatoes reach a certain green mature stage, the plant starts producing a gas called "ethylene". Ethylene interacts with the fruit to start the ripening process. Excessively cold (~< 50 F) or excessively hot (~> 85 F) air temps and/or consistent winds (that can carry-off the ethylene gas) can slow and even completely stop the ripening process.
Try waiting a bit longer. If you are unable to wait, pick them green and place them in a brown paper bag for a few days (the bag traps the ethylene gas thereby accelerating the ripening process.