There are lots of reasons it could be happening. One reason is wilt
that is very common in tomatos. Some varieties are specifically bred
to resist wilt but many are not. If your varieties are not VFN
resistant, there is not much you can do about it.(Assuming it is a
wilt problem).

Bob S.

Bob, you will find that vascular wilts appear more like the plants
need a drink as the pathogen clogs up the the stems. A simple test is
to make a long slanty cut in the stem and touch them together, when
you pull them apart you see sticky strands and if you look at the
vessels you see discoloration.