If there is peat moss in the mix you used in the container, the problem is probably low pH. Many mixes tend to be too acidic for spinach. Spinach, Okra, and beets require a pH of about 6.5. If you add some Ag lime is will slowly raise the pH. Hydrated lime will work faster but it is caustic (will burn leaves). The lime should be mixed into the soil as it moves through the soil profile slowly.

I had the same problem any mix that uses peat moss. Peat moss has a pH of about 4.5.

Also, spinach is notoriously hard to grow in FL. Chard is about a thousand times easier and makes a good substitute.

Spinach is all around not easy to grow. It bolts very easily and is finicky about soils, as per previous posters. The yield is also not really worthwhile unless you are eating it raw. I prefer to grow beets and chard they are a bit more flexible.

Thank you everyone for your replies. I will do a little more research on soil conditions and attempt to make a nice container planting in the spring. Thanks once again for the replies.