How often are you Watering?
There seem to be a fine balance between
too much and not enough.

Is your bed drained well?
Maybe too Well?

I plant in ground, well drained soil, mulch heavy,
and this time of year water 5 days slow and deep.

We even had 102 temps last week and all was well

Here is a link that might be useful: Watch my Tomato Growing Video series