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 (the LaserJet 4250 may be too old for this) 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).