I am no expert.... But just how 'moist' is the soil underneath? It looks like it maybe could be that the roots are kept too moist. Does the frame you have on the concrete have really good drainage? Have you had a lot of rain?

And unless it is an artifact of the photo, that soil looks really rich and black. Tomatoes like this. Beans, less so.

Thanks for the reply! :)

The soil is just moist, not too wet - there is no standing water, the boxes have good drainage.

I am having the same trouble with my beans. I thought it was overwatering so I have let them dry out for just over a week. my corn is looking much better but the beans are still very yellow. I planted them in 70 30 compost topsoil mix, so could it be my soil? If it is too rich, how do I make the soil less "good"?