As you know, VR5 have support the max bandwidth with 100MHz, when the input port try to make the power measurement, it will take 100MHz bandwidth measurement and give the result.
If the signal bandwidth you are using is lower than 100MHz, there will be additional noise in the measurement result and sometimes cause no measurement if you are using TDD signal.
We would suggest you to do accurate signal measurement, you will need to fine tune below parameters
From VR5 GUI:
1, Power Meter Settings:
2, Advanced Port Settings: