Nothing is happening in my compost
Most ‘slow’ piles are working exactly as expected. Cold composting takes six months to two years. It is only a symptom if the pile is dry, starved of nitrogen, or full of material too large to break down. Below: what causes compost decomposing slowly, how to fix it today, and how to stop it happening again.
First, is it actually slow?
An unturned heap you add to gradually takes a year or more. That is the normal outcome of that method, not a failure. Compare against how long compost takes before troubleshooting.
The three real causes
Too dry. The most common. Microbes need water; below about 40% moisture they go dormant.
Not enough nitrogen. A pile of leaves, straw and cardboard alone will sit for years.
Pieces too big. Whole branches, thick stems and intact vegetables have very little surface area. Chopping is the highest-leverage thing most people never do.
The stubborn things
Avocado stones, mango pits, corn cobs, eggshells, citrus peel and anything woody outlive several cycles. Sieve them out and throw them back in the next pile. This is normal, not a fault.
Related
Index of symptoms: compost problems.
Further reading on compost decomposing slowly
Independent sources worth reading alongside this page: EPA, Composting at Home · Cornell Waste Management Institute.