You should also check your propane pressure, or have it checked. I made a manometer (Google is your friend) and found that my pressure was WAY low and dropped even further when the furnace came on.
In order to get a good flame, I had to open the air shutter too far. Low propane pressure could cause the flame to recede to the shutter.
You should have 11" water column (about 1.5 psi) at the heater with two other appliances running. I tried to adjust my regulator (2002 vintage) and it went from WAY low to WAY high. I replaced the regulator.
"You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO
2002 Classic 30 Slideout
2001 Safari 25 RB Twin (Gone, but not forgotten)
2013 Dodge 2500 Laramie 4x4 Megacab Cummins
WBCCI 1322, TAC FL-39, AIR 82265