What are you using for a thermowell (material)? If poly, not a great conductor and could be an issue especially if not a tight fit. I agree on no copper. I use the ss ones from Brewers Hardware. The probes do not fit snug but the thermowell is a good distance in the liquid and ss is an adequate conductor so the temp inside it is pretty much on. I have the surrounding temp up to 10 deg colder than fermentation temp and no issues. If questioning the probe, test it by having it next to a proven temp device at various temps (fridge, freezer, ale box, room, other). Not a perfect calibration but it will tell you if you have a serious probe issue.