I just went around and around USPS with a similar problem. Seems as though I had taped across the scan lines, and the tape had a wrinkle. The reader couldn't read the scan lines because of the wrinkle, and evidently it was too much trouble to read it by eye. So, it was returned to me. I couldn't re-print the label because the tracking number had been used, but I re-sent to the same address (with no tape) and it was delivered fine.
USPS does say not to tape over the label, so I suppose it was my own fault, but really???????