really the wooden doors, lack of any gas residue, lack of any bodies, inability of the furnaces to burn even a fraction of the bodies they supposedly did, 'gas chambers' that had windows and/or ventilation connecting to the officer's barracks (depends on the camp), chimneys that were built years after the war and don't connect to anything. jewsish population data showing an increase in population, why would the Germans transport prisoners between camps if they were just going to kill them? all of these are much better reasons than 'not enough concentration in the gas'