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Sounds like they need more light. Are you growing indoors or outdoors? Indoors, I would increase the amount of light and move the lights closer. If it is outdoors, the area may be too shady.

Here is some information that may be helpful to you: http://www.nutrition-and-you.com/endive.html

Compost or manure. This article has other suggestions: https://www.gardeningknowhow.com/edible/vegetables/vgen/fertilizer-options-for-your-vegetable-garden.htm

When it gets dry. Too many variables to answer your question...heat, humidity, sun, wind, size of plants, soil type and consistency as well as amount and frequency of watering are all factors determining the answer.

There are many online seed sources. Enter "buy endive seed" or "vegetable seed" into a search engine.

Generally, if you allow the plant to flower, it will produce seeds, which you can harvest and save for next season. That said, you can also look for the type of lettuce you have (should have included label) or simply look for any of the leaf lettuce varieties - all of which are easy to grow. Seeds should be available at most garden centers. This article should help as well: https://www.gardeningknowhow.com/edible/vegetables/lettuce/growing-lettuce-containers.htm