HP 3550 - Manual (Page 189)

Prints on one side even when 2-sided (duplex) was selected
Cause Solution
Manual duplex is not enabled. Select manual duplexing (2-sided). See Manual
2-sided printing .
The size of paper selected does not support 2-
sided (duplex) printing. Load and select a size and type of paper that
supports automatic 2-sided printing.
Paper handling problems (continued)
ENWW Paper handling problems 177