McGiever wrote:Like Tomcat above says check under the grates for accumulation of 5 years of fines.
And if not already mentioned, check/clean horizontal pipe sections and the base of chimney...flyash settles there and can choke off most of the pipe opening inside.
To add to what McGiever has said, is your stove pipe shoved too far into the chimney. I've seen were stove pipe has been pushed into chimney, all the way to opposite wall of chimney. This would restrict / block exhaust gasses from leaving pipe. Depending on pipe set-up, it'd look the same, pushed in to far, or not. Oliver