Please explain my looper problem

[moro]

This is one of those things that makes me believe that I will never understand electronics.

I just built a looper with 2 FX loops and a tuner / mute. I've built one before with no problems and aside from a few wiring changes, the big difference between my last one and this one is that I put a 1M resistor between the tip and ground of the  send / return jacks. I only had four 1M resistors, so I didn't put one on the tuner send.

So I finish it, plug it in, and it works except for the tuner. It mutes the signal, I have continuity when I check it with a DMM, but the tuner doesn't respond. I manage to scrounge up another 1M resistor, so I put that across the tip and ground of the tuner jack. I also notice that I forgot to short the output to ground when the tuner is on, so I fix that too.

And when I plug it in, sure enough, it works. What's going on here? Can someone explain this to me?