hard to tell the problem without knowing the plant species, it's location in the yard, your watering schedule, the size of the planters, and the planting medium being used. but just from what you've mentioned so far you're probably right, they're probably not getting enough water where they need it. large planters are normally used for plants that need more root space. one way to raise the water level in your planter though would be to fill the space with something other than planting medium. if the planters are very large, you can easily fill them with bricks, gravel, cheap dirt, pavers, broken concrete, etc, then put your annuals in a smaller pot which you would bury below the surface of planting medium of the larger planter. this saves on expensive gardening soil as well as keeping the water where you need it, but retains the look you want. HTH

Another option, along the same lines, is to plant your annuals in much smaller containers and place those in your planters. Cover everything up with lots of mulch/bark. That should help retain water near the rootballs.

Both great ideas. I will try and plant them in small containers and see if that works

You've got great suggestions for different approaches, do keep us posted.

You know, despite the summer heat here, I just don't like containers without holes. I think they do just what you describe: too wet at the bottom, too dry at the top. At least with a container with holes we can water often and the excess drains out instead of pooling and possibly fostering bacteria or something.

Super good luck and do keep us updated. I cut back a lot on containers in summer, but I still do quite a few filled with vincas, hibiscus, portulaca etc., plus several hundred cacti and succulents too of course.

Good luck!
Grant