LFE silent..?

Greetings all,

I have many 5.1 mixes in which some appear to route fine to LFE, some don’t. The busses are the same, the LFE knob is tweaked from 0 to +10 sometimes just for testing, on both the master fader and the channel surround panner, even converting the clip to mono doesn’t do anything. The LFE channel is silent. Any ideas…??