What may be the case:
- With many modern printers (and probably your model), 'sided' paper (like Letterhead, or Prepunched) has to be loaded in the cassette trays in one way for Simplex print jobs, but a totally different way for Duplex print jobs.
A possible circumvention:
- Some printer models support Alternative Letterhead Mode, which allows the paper to be loaded the same way for all (duplex and simplex) print jobs - I think the printer achieves this by adding dummy rear face pages to simplex jobs, and duplexing them (so there will be a slight throughput reduction with simplex jobs).