6100LM owners...have you had this problem?

[alvagoldbook]

I was messing around on my 6100LM earlier today, and I noticed I had some issues with the effects loop. It seems like it's not working at all in Series mode, only in Parallel.

 

[Keeb]

I had this problem (non LM version though). One of my trimmers for adjusting the channel levels was stuck so I used a screwdriver, some cleaning spray and some force and when it came loose it all came to life.

So yeah, check those trimmers. Good luck!