Note that the maintenance problems have got nothing to do with AC or non-AC. In fact, maintaining AC coaches is more difficult than maintaining non-AC ones.
Higher speeds require more rigorous maintenance (this is not about having later technology, it's just a physical limitation), which means that the staff have to spend more time on maintenance for each rake. This is a problem for depots handling maintenance for many trains. So, they tend to prioritize the more important ones, usually fully A/C trains.