Canon MX720 Series Online Manual page 156

1310
Cause
The size of paper may not be compatible with automatic duplex printing.
Action
The sizes of media suitable for auto duplex printing are A4, Letter, A5, B5, and Hagaki. Make sure that the
size of the paper loaded in the machine is correct. Pressing the OK button will eject the paper and restart
printing from the front side of the next paper. The reverse side of the ejected sheet will not be printed.