Next time I will start by suggesting a double-check on the feed and air settings.
It is easy to skip over the simple things and assume they have been checked. Anyhow, good troubleshooting on your part. I'm sure it will take on a new attitude after your adjustments. Please report back with the results.
Also...if he is burning rice coal, you may be able to back the air down a little more. Next time you go over put a load on the boiler and after 30-40 minutes of stoking take a look at the ash ring.