drought tolerant is a relative term. all plants require water. hanging baskets are much more dependent upon being watered than plants in the ground. and verbenas need more water than lantanas. if you see no signs of disease or pests, then you are underwatering the verbena.

I always have to water any hanging basket daily.

I agree, I would never go without watering a container in full sun,every day, unless we were having a lot of rain and cloudy weather and then I would check it almost every day just in case.[g] We had a heat wave here this week and I missed one container on my watering schedule in the morning and when I went back out in the afternoon, every plant in the container was wilted and had fallen over. It was my favorite container. I soaked it right away and it has come back but isn't quite as pristine as it was.

About the only thing I can think of that you can put in a container and not have to water every day would be cactus and sedums and the like. Are you checking for moisture on the days you are not watering? You might try some mulch on the top of the soil to conserve moisture and/or put the container in less sun. My containers that only get some morning sun and are planted with shade plants, are having to be watered a lot less than my other containers.

For full sun hanging baskets you need lots of foliage covering the basket. Full sun hitting the basket area is NOT GOOD, no matter how often you water it. (Only roots I like boiled are beets, carrots, potatoes etc:)

Trailing Verbena is not usually compact enough to do this on it's own and needs some maintenance (snipping the leads) to make it fill in.

Inter changing baskets from full to partial sun will help too, giving the plants a break from the heat.

Lastly they could run out of food, so fertilize with a liquid fertilizers about every 2 weeks..