My p43 will light off on the left side only after sitting for a few hours. Sometimes it will spread to the right but most of the time I have to mix it up to get a full burn. Funny thing if it only sits about an hour it appears to light correctly. This started last spring and has continued into this fall. The vent is squeaky clean and so is the stove. I've pulled and checked the igniter for crud blocking the airflow and found none. Any ideas?