I had the same issue with my AP rack (economy rack for iris 41qt). I dremeled out the pre cut channel so I could tape the probe directly to the flexwatt (also using a herpstat 2). I'm using precut paper liners, but I have to have the herpstat set at 94 to get temps of 89-90 in the rack. It's been consistent at this point for several months so i've kind of chalked it up to just 'one of those things.'