There's a couple of possibilities, both have to do with polarity. (what some call phase reversal)

a- There's a good chance that one of the two cables that you have is wired incorrectly. (tip on one end wired to the sleeve on the other end)

b- One cable is a TRS cable while the other is the typical TS

c - Cable is not pushed all the way in (ck all 4 connections)

d - Some other anomaly based on conditions yet to be discovered.

(actually, I guess that's more than a couple)

Oh, and to answer your question as to why the bass disappears. Anything that's common to both channels (ie, anything panned to the middle) get's canceled out by the reversed polarity when panned to mono. When in stereo you can hear everything, but it probably sounds a little weird. Sort of super stereo maybe. Because of the reversed polarity on one of the cables, while one speaker is pushing forward, the other speaker is pushing backward.
Your lesson for today is to research "Phase Cancellation" "Phase Reversal" "reversed polarity"

keep us posted.

What kind of mixer? Do you have a phase inversion switch on one channel?

BobRogers wrote: What kind of mixer? Do you have a phase inversion switch on one channel?

oh yeah, I guess that would be [d]

Sorry for being MIA.

Thank you very much for the information.

The problem must have been the wiring in one of the cables but I do not have that specific cable anymore so I guess I'll never know.

It works fine now though with different cables.

So from what I have learned I'm guessing that the one cable what wired incorrectly so it was off phase with the other cable causing phase cancellation.

THanks,
Kurt

p.s. mentioning TRS and TS is also helpful for the future.

Actually, you had inverted polarity, which is different than something being out of phase. Two signals out of phase can have the same polarity, but not the other way around.

Just so you know.