Here’s what appears to be wrong with your basil plants:

• Too many plants per pot.
  At best … one plant per 6″ pot and use a deeper pot.
• The leaves show signs of downy mildew.
  - Spray with a baking soda and soap solution before signs of leaf damage appear.
  - Avoid watering from the top. Set pot in tray and water from bottom.
• Pinch the top out of the plants when they’re ~6″ tall to increase production and again when the new branches are ~4″ long.
• Pinch off all flowers to prolong production.