OHH I bet this is the same question I just posted to. So it is a forced air kerosene. The usual is like I said a blocked combustion air hole. It could also be something shorting out the spark ignition to ground when replaces inspect the wires and look for a week or loose ground in this case. Sorry about the confusion.

A Bad ignition proving circuit or something like (what we call here dirt dobbers) hornets nest could be letting the flame hit it and not allowing full flow through the heater. Mice are also notorious for making problems like wires chewed and nests in the wrong place. Given where it may have been stored.