I have a routing problem. I'm trying to sidechain compress bass guitar using the kick drum as the source. The routing as it is:
Kick -> Drum Master BUS -> MASTER
->POST send to MB LP (ext switch on)
Bass DI + Bass Amped -> Bass Master Bus (MB LP on the FX rack) -> MASTER
 
I tried a similar scenario in a test project without any busses (kick track sidechaining LP MB in a bass track) and worked as expected. In the actual project nothing is happening and the sidechain audition in MB LP is silent. What am I doing wrong? 

I still don't know why that doesn't work but here's a workaround if somebody counters the same problem:
Make a send from kick track to a new BUS then route the output to MB LP sidechain.