I see a lot of interveinal chlorosis, perhaps Sulfur or Magnesium deficiency?

What is in the Earthbox, potting mix or actual earth/soil?

Have you ever fertilized the plant? It is quite large and fruiting, so it might have depleted whatever nutrients were in the medium. Since it's a container with little microbial activity, you need immediately available nutrients, such as liquid fertilizer in the water.

Promix potting mix...same stuff as my "successful one". I fertilized initially as per the instructions then gave it fish emulsion, kelp, and worm tea in water to try to perk it up. I would hate for it to die. It has so many tomatillos. :(

Annie - did you ever resolve the problem? (I know the original post was 4 years ago - but I'm having the same issue with one of my plants now)