Never try growing spinach from store bought seedlings. Spinach hates being transplanted and will quickly go to bolt. Always directly sow seed where you plan to grow your spinach. The next lesson about spinach is always sow your seed in an organically rich soil. When we did get around to sowing seed the soil was very clayey and poor in organic matter.
Spinach loves an organically rich, free draining soil.
Otherwise you'll get poor, stunted growth like we did. Feed them fortnightly with liquid manure to encourage leaf growth. And the final lesson which you can learn from our mistakes is to always use fresh seed. After getting all of the above right we used the same packet of seed. Our spinach seedlings were as pitiful as ever.