Not enough light

What's happening

Rex begonias produce their striking silver, pink, and burgundy patterns in response to good light. In a spot that's too dim, the plant can't power that vivid coloring or robust growth, so new leaves come in small, dull, and widely spaced, and the whole plant looks flat and listless rather than rotting or wilting.

How to fix it

Move the plant to a brighter location with plenty of bright, indirect light — a few feet back from an east or north window is ideal. If your space is genuinely dim, a full-spectrum grow light on a timer makes a real difference and is often the only way to restore the rich coloring. Already-faded leaves usually stay dull, but new growth in better light will return brighter and fuller.

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