Mint

Wilts flat by mid-afternoon, recovers after a drink, does it again the next day

Mint grows on wet ground at the edge of streams. There is no drought tolerance built into it at all. A pot full of roots dries out faster than the plant can cope with.

Fix: Move it into a wider pot with fresh mix, or split the clump and repot half of it. Standing the pot in a shallow saucer of water on hot days is one of the few times that is good advice.

Fine pale speckling across the leaves, webbing in the leaf joints

Spider mites, which do very well in the dry warm air of a heated room.

Fix: Take the plant to the sink and spray the undersides hard with water, repeating every few days for a couple of weeks. Cutting the whole plant down to a couple of inches and starting again is often quicker than nursing badly infested foliage.

Everything else in the pot is dwindling

Mint spreads by runner and has no manners about it. Given a shared container it will own the root space inside a season.

Fix: Give it a pot of its own. If it has to go in a mixed planter, sink it in a smaller pot inside the big one and lift it every few months to cut off any runners that have escaped.