When PMP 450m is not sounding, a few things to check:
Home Screen
Feature Key Information
MU-MIMO Mode: MU-MIMO
Radio Configuration
Advanced
MP 450/430 Legacy Mode Disabled
If the PMP 450/430 Legacy mode is enabled, the Statistics --> Sounding Statistics will show Sounding State UNKNOWN for all SM's, MU-MIMO Rate Not Eligable.