The person who notices a quiet improvement should say so before the next complaint becomes urgent.

Feedback tends to arrive only when something is wrong. If the only time anyone hears from you is when the report was late or the room was a mess, then your attention becomes something to dread, and the effort someone made to fix things last month goes entirely unrecorded. Improvement that nobody mentions is improvement that stops.

When you spot that something has got better, say it within a day or two. Keep it plain and specific, and do not pair it with the next request. A person who has been noticed for getting something right is far easier to talk to when the next problem comes.