Sounds like a root rot issue from overwatering or perhaps salt build-up from fertilizers and/or irrigation water. Are you watering so that at least 10% of the total amount of water applied appears at the drain and is discarded?

High salt content raises the EC (electrical conductivity) of the soil and makes it difficult for the plant to take up water & dissolved nutrients.

Shading the container to insure soil temperatures remain below 30* would also help insure efficient root metabolism. 30* is the approximate temperature at which root function begins to become seriously impaired.

Al