Likely the heat - if they're in sun.

Al

Carol, what are the plants? How many drainage holes and how large are they? Does water run out of bottom of barrel at all when you water? Drainage holes can become clogged with roots sometimes. To check moisture stick a bamboo skewer or thin dowel into soil and leave for a minute...is it dark with moisture? Sometimes barrels dry out too much and soil pulls away from sides...then water just runs right thru without soaking into soil.

Sorry for all the questions but it's hard to attempt to diagnose without more info. Can you post a little more info? Al may be right that it is just too hot if in sun but my sun is fiercer yet I have lots of wooden boxes in sun. josh

I've just logged on here for the first time, so didn't really expect this to appear!
I have one more question, though, besides the ones Josh asked: do you have mulch covering the soil in the barrels?

Salinity in container plants can build up if they are not properly drained. Nurseries prevent this by allowing about 20% of the water to drain out. It hard to say this is the problem without some detailed symptoms or plant type. The basic watering procedures should be: water until all soil is saturated and some excess is drained out, don't water again until the top of the soil is dry to the touch. This would cover most plant types.

Just in case it was a drainage problem, I had DH lift the barrels up onto a 2x4 platform. I watered until I saw the water begin to drain out the bottom. The plants already look better. It was probably the drainage and watering problem. My other barrels are sitting on the ground so they should be draining into the ground. I do think I need to fertilize them now.