One question. Do you have TFC enabled? You can enable it by pressing the TFC knob on the EFIS panel. If it is enabled with the EFIS knob you can see TCAS OFF in yellow/amber on the ND (when XPNDR is STBY), like here:
If the XPNDR is in mode TA you will see TA ONLY in blue.
In mode TA/RA it's TFC in blue.
Location of the TFC button on the EFIS control panel:
Also know that PMDG TCAS traffic is preset to display only traffic at + and - 2800' of your altitude at a max range of 40 nm.
You can alter the parameters through the CDU -> PMDG OPTIONS