tomato flower heads are falling off

You may be doing nothing wrong. Temperature is the most common cause of flower drop in tomatoes, and if your weather was anything like mine this year, that may be what's happening.  Here are the stats on that;

Above 85 F daytime temperatures and below 55  F temperatures at night tomato blossoms do not like.

Tomatoes grow best if daytime temperatures range between 70 F / 21 C and 85 F / 29 C

There are some varieties that are a little more resistant to the heat, you would have to research that as I don't know off the top of my head which ones they are.

I don't know why they are this way but extreme temps keep them from pollinating. Tomato blossoms usually polinate themselves as they open, so it may help to shake the plants as the blossoms are getting close to opening.

p.s. This self pollinating habit is what makes it really easy and safe to keep seeds from "non-hybrid" plants. They very seldom will cross pollinate even plnted as close as 4' apart.