If the RAM stick, in the case of a Notebook e.g. SO-DIMM, is either 'bad' or improperly seated then sure it won't show-up to the OS. It may still show-up in CPU-z but if the BIOS deems it 'bad' @ post then it will be turned-off to prevent errors.
First, it's bad to mix-match RAM (not from a kit), I would suggest that you remove the RAM and try (1) stick at a time, and if (1) fails then chances are it's bad -- you can try it in a different DIMM slot. Most RAM carries a 'Lifetime Warranty' so I'd contact the OEM and request a replacement.