Flies in my compost

Fruit flies (small, hovering, everywhere at once)

Attracted to exposed fruit and vegetable waste. Harmless to the process, genuinely annoying to you.

Fix: bury every addition under at least 5cm of browns. Keep a bag of dry leaves or shredded cardboard beside the bin. The population collapses in under a week once the food is covered.

House flies and maggots (large, in numbers)

Usually a sign that meat, fish, dairy or cooked food has gone in. The maggots are typically black soldier fly or blowfly larvae. Soldier fly larvae are actually efficient decomposers and not a health problem, but their presence signals material your pile cannot handle discreetly, which is also what attracts rats.

Fix: remove the offending material if you can find it, cover deeply, and keep those materials out.