Thermo coupler replacement on ventless gas logs?

My pilot light on my ventless propane gas logs will not stay lit after releasing the pilot button. Is this always the thermo coupler? Or could it be dirt or water in the gas line? I've been told that spiders are drawn to propane gas and build "nests" in the lines. Is this true? If so will blowing out the line with compressed air help to free up the line. Although I would think that if the pilot lights the line is clear.

Len McLaughlin
October 2009