I load a Smoke Tube, I've used both pellets and wood chips, light it and let it burn for several minutes, smoke billowing out of the tube. Lay the tube horizontal on the grill, let it burn for a few more minutes and then close the lid. I see smoke coming from the Traeger, but only for a few short minutes. I don't think the tube is getting enough oxygen to continue to burn and produce smoke. I have an insulated grill cover which may also limit the amount of oxygen needed for the tube to burn.
Bottom line, smoke tube doesn't really seem to work.