As I know, su27/su33/mig29, if you active your radar, there will be a character like inverted 'N' on the left of your HUD. Check that...
when radar's on
you can get IFF messages only from HUD (not HDD/MFD) when you are using BVR-SCAN mode
you can get IFF messages from both HUD & MFD when you are using BVR-TWS mode
you can get IFF messages from both HUD & MFD when you are using BVR-STT mode (lock target)
you can get IFF messages only from HUD when you choose CAC mode (VS, Bore etc), and you can't get anything from MFD then.
For more detail, refer to your LO1.1 manual...