Tomato Problem

I growing a few varieties of tomatoes - brandywine, mellow yellow, roma, beefsteak, tiny tims. All of them are fine except the romas. The romas will get a nice blush then fall to the ground. So far I've managed to save them for ripening on a windowsill but why are they doing this? They are nice size and the plants look quite healthy. They are loaded with tomatoes. I just can't figure out why they drop from the stems before fully ripening. Any ideas?
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