Gas Furnace not Venting Properly

home inspector said gas furnace is not venting properly. it's an older furnace that doest have the prepurge fan. aside from the vent being screwed up, what else could cause this problem? TIA
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Mr. Whatever wrote:

Birdnest or raccoon in chimney. Chimney collapsing. Snow blockage.
Jim
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Do you have a something else in the same vent, like water heater?
--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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Ask the inspector he has to be specific and detailed, or do you want us to guess? It could be 7 things that come to mind, ask him..not us.
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Joseph Meehan wrote:

nothing else tied into vent. no obvious problem with vent piping. i'll check from roof after the rain stops.
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