I had the same problem with Chard. Asked the question in Vegetable forum yesterday, and got a response that lime was needed. It is very acid here. Are you acidic there? Maybe same issue.

Mustard greens grow great in our climate. They are very good picked fresh and cooked immediately. Do you know how to make hoe cake? Any corn bread would be good with them, but they do require corn bread.

Mustard greens and lettuces are the easiest to grow. Scatter the seed, but don't cover with soil. They need light for germination. I top them with a very thin layer of pine straw.

I used to grow Swiss chard from seed, but it seems to take so long until it's ready to harvest. Seed needs to be soaked overnight. Now I buy transplants. Much easier and I harvest much sooner.

You can still sow mustard, lettuce, and chard.

Here is a link that might be useful: LSU veggie planting guide

All cole plant do well in our area, as well Chard and lettuce. I prefer Butter Crunch head lettuce. Sweet taste but it takes a whole head for 2 people. Mustard greens do fine in cool weather. I prefer all to be directly seeded in the soil, win, lose or draw.