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I really don't know unless the seeds are not viable. While these can be finicky when it comes to germination, you should still have gotten something. And since you've been gardening this long, I would think the soil and growing conditions for this plant are being met. Very unusual. I wish I could be of more help, but all I can think of would be the seeds or the soil. You may want to try a soil test to see what this turns up.

Harvesting of greens can vary between variety, but you can harvest most greens by taking individual leaves, cutting the plant at the soil or pulling the entire plant.

https://www.gardeningknowhow.com/edible/vegetables/greens/leafy-garden-greens.htm

Fish emulsion will be, mostly, nitrogen. This deficiency will appear on the newest leaves first.

Because of the high nitrogen content, a weekly feeding may bring the soil to be too acidic. This can lock out phosphorus and potassium, as well as calcium and magnesium.

That looks like the classic symptoms of a magnesium deficiency.

I would add a simple handful of DOLOMITIC LIME per container. This will help to correct the issue.

In the future, I would make sure to test soil EC (electrical conductivity) which will tell you the nutrient content, and the pH before adding nutrients to make sure that there isn't already enough or too much.

Alternatively, you can do a dropper test, which can tell you some of the same information. This article will help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

This article will help you to grow winter greens:
https://www.gardeningknowhow.com/edible/vegetables/greens/growing-greens-in-winter.htm