Ah, the classic stereo-to-mono problem(s) we all experience at some point - our sounds disappearing!
Basically - you're experiencing phase issues.
For whatever reason, stereo sounds pertaining to you virtual instruments has caused them to kind of... cancel each other out.
...I'm not sure if I'm allowed to link to other forums on this site, so I'll just copy & paste some responses:
"Phase compatibility. If a mix has tracks that are partially out of phase (left vs. right) in certain frequencies, when summed to mono it will partially null in those frequencies."
Here is some other recommended reading material.
Since you don't have it panned, it's more than likely an issue with whatever synth/piano patch(es) you're using.
Try to have a look around in the patch settings and see if there are any sort of stereo effect (a "width" or "space" knob or something like that) that's causing it.
Unfortunately I'm not experienced as the others here when it comes to troubleshooting from afar so I'm not of much help. People who know more than me will be here to help soon. I'm looking forward to what you all have to say too btw (so I'll know what to do if/when I run into the same problem)
