Pyris 1 TGA problem while using oxygen as gas for sample environment

Whenever I use oxygen gas for my sample at low heating/ramp rates (5 degrees C/min), I fail to get the expected curve (showing drastic mass loss at oxidation temperature), rather I get the same curve as I get when I use air. However, at higher heating rates (10, 15 and 20 degree C/min) I am able to get the correct curve. I changed the oxygen cylinders, oxygen pressure regulators, I have check the inlet pressure and inlet flow rates too. everything is in rage and normal. Any insight would be greatly appreciated.

Thanks.