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?