Most coffee-compost problems trace back to the same root cause: too much wet, nitrogen-rich grounds and not enough dry carbon to balance them, covered inthe ratio guide. Here is how the specific symptoms map to fixes.
It smells like ammonia or sourness
This is a nitrogen surplus. The pile is short on carbon and short on air. Mix in a large amount of shredded leaves or cardboard immediately, and turn the pile to reopen air pockets. The smell should fade within a few days once the ratio corrects.
It is a wet, gluey clump
Coffee grounds are fine and dense, and on their own they compact into a layer water cannot drain through. Break the clump apart by hand, mix in dry browns until the texture crumbles rather than smears, and stop adding grounds until the rest of the pile catches up.
White or grey fuzzy mould on the surface
A thin layer of white mould on fresh grounds is normal and harmless, it is part of the breakdown process and will be consumed as the pile heats up. Thick, spreading mould paired with a wet, matted texture is the compaction problem above, fix the ratio and airflow rather than the mould itself.
Nothing is breaking down at all
A pile that just sits there, not smelling, not shrinking, is usually too dry rather than too wet, common when the browns-to-greens ratio swings too far the other way. Add a fresh layer of grounds and a little water, then turn.
Fruit flies or gnats around the bin
Exposed grounds and food scraps at the surface attract flies. Bury fresh additions a few centimetres under existing material, or top the pile with a layer of dry leaves or cardboard each time you add something wet.
Once the pile is balanced, coffee grounds actually help: they attract earthworms and add nitrogen that speeds up the whole process. The goal is not to avoid coffee in compost, it is to never add it alone.