Plethora midi thru

New

Midi thru issues
I have a plethora (chnl 4) and strymon iridium(chnl 2) also a midi to trs cable ( non strymon)


I can control,the plethora . I can control the strymon direct connecting the midi interface to the trs port of the iridium
But
If I use the plethora midi thru port to reach the iridium then I cannot control the iridium or apparently make any contact


Ideas?