Chestnut requires a somewhat loose fast draining, acidic soil to thrive. It's hard to tell from a photo, but the medium in your pot appears finely textured. If the PH of your soil or water are too high, this may be a difficult plant for you to grow. High for chestnut would be neutral (7) or above. Better for chestnut is PH in the 5.5 - 6.5 range. Planted in soil not acidic enough, or watered from a source that is neutral or above (alkaline), the plant is unable to utilize nutrients in the soil, i.e. iron and magnesium resulting in 'chlorosis'.

perhaps your chestnut is sad because it really wants to sink it's roots into some nice ground and be liberated.

Curious, is that an American, Dunstan, or Chinese Chestnut? Just like any bonsai, you must replenish the potting medium yearly. I agree with the previous post that your soil looks like compacted mud/clay. Roots need air. I'd repot. If you are into growing chestnuts check this site out.

https://www.qdma.com/forums/showthread.php?t=46975