First thing I would suggest is seeing if the problem is with the PDM meter or Dexcom.
Very first thing is to see if your PDM meter is accurate. A few easy things to try.
1) Maybe you are already doing this, but make sure you are washing and DRYING your hands before tests. Traces of water, sweat, or food on your fingertips will mess up your readings.
2) Take several tests, a few seconds apart, on the PDM meter. They should be very close. How close are they?
3) Use a different meter, and compare it to the PDM meter. How close are they? If you don't have a second meter, you can buy them for a reasonable price. If you take tests with both meters at the same time, how close are they?
If your meter is not accurate, and your PDM is under warranty, you can get a swap from Insulet for free.
If you have an accurate meter, then it's a problem with your Dex. There are certain things that make it less effective for me. Sweating a lot makes it practically unusable, until I have replaced the fluid. It totally flakes out on me with a lot of sweat. Placement of the sensor, and your tissue type will also affect how accurate it is. My tissue type doesn't work well!
If you find it is a problem with Dexcom, reach out to rgcainmd on this forum. She is a Dexcom maven. A connoisseur in the art of CGM.