Underwatering

What's happening

Yucca tolerates drought longer than almost any houseplant, but it isn't invincible. When the soil stays bone dry for weeks on end, the plant can no longer keep its oldest leaves hydrated, so those lower leaves yellow, soften, and curl while the soil shrinks away from the side of the pot.

How to fix it

Water thoroughly until it runs out the drainage holes, then let the soil dry out fully again before the next watering. If the water races straight through bone-dry soil without soaking in, bottom-water instead: set the pot in a few inches of water for 20–30 minutes, then drain completely. Going forward, check the soil every couple of weeks and water deeply once it's dry several inches down — yucca much prefers a deep soak followed by a long dry spell over frequent sips.

This is general guidance based on common symptoms; individual plants vary.