To answer your question much more info is needed. Just to say my plant is wilting gives no one a hint of what the cause is. A wilt can caused by several things.

Are these recent transplants?
Do you have a lot of high temps.?
How much do you water--how wet is the soil--do you let it dry between waterings?
What exposure are they in, North, south,etc--
Is there anything else shading them. How many hours of sun and the time of day it gets the sun.

I doubt it's disease. Snapdragons go wilty when they need water. Window boxes dry out quickly - make sure your watering reaches the roots and isn't just wetting down the soil surface.

I don't think it's lack of water because the other plants in the planter are fine. It's a western exposure, about six hours of sun/day getting both morning and afternoon sun but shaded in between.

I do let it dry between waterings and we haven't been getting a lot of rain, maybe two or three times/week. They have been planted for about four weeks. We are going through a hot spell right now but when I first noticed the problem it was in the 70's and lower 80's, not that hot.

I hope that's enough info. I will try to post a photo tomorrow. Thanks again!

I'd pull it up and discard. Snaps are prone to root diseases such as pythium. The plant may have come to you already infected or have contacted something when planted. Be sure that container mediums drain rapidly, even if you have to water more often. A fast-draining soil is very important for plants like snapdragons.

Thanks rhizo, that's what I'll do. Should I remove all the plants and soil and replant completely from scratch?

Keep an eye on the others...they may miss becoming infected with whatever the pathogen is. That's often how it goes with snaps.