If you are watering primarily in the cup, and supplying a dribble here and there to roots. All fertilizer salts are accumulating in the grow medium, which makes it continually more difficult for the plant to take up and move water efficiently. Eventually, as the level of dissolved solids/ salts in the grow medium continues to increase, water absorption will become impossible, or even act to pull water out of cells, reversing the normal process of osmosis and pulling water OUT of plant cells via the same mechanism curing salts 'pulls' water from the cells of meat destined to become bacon, jerky, ham, ..... . The technical name for what is occurring is 'plasmolysis', but more commonly it's known as fertilizer burn.

The fix: Flush the soil on a regular basis. The grow medium looks very dense, so you might need to use something that allows you to flush the soil regularly w/o the need for concern the medium will remain wet too long and cause root issues related to either function or health.

Al