Well said Dave.
I was just going to say: "Watch the polarity".

My leads all have one of the mono jacks labelled 'Tip' or with a coloured sleeve.

I have desks by Allen & Heath (which are Tip send) and Soundcraft (which are Ring send), so always have to watch out they are plugged the correct way.

If you have a channel which is 'dead', take the insert out and sound should be restored if their is no other fault. Swap the leads on the outboard device, re-insert the stereo jack in the mixer, and all should now work. An easy way to check is to see if there is any signal going into the outboard device (most have input meters/LEDs). If there is, at least you know you are sending signal correctly.

Hope this helps.

G