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?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.