The 3335 is an ADF (Single sided) and the 3345 is a DADF (2 sided)
So assuming you have a 3345, I would assume 1 of 2 scenarios.
1. You can do only 2-1 (2 sided original, one sided output) because you chose a stock that can't be duplexed
2. The DADF has a hardware fault (Hopefully rdaniels2 or Gabiene will chime in with suggestions for that)
Are there any faults displayed on the device?