mono mixing

[SYN420]

Im not sure if this has been discussed, I didn't see anything about it yet or maybe I overlooked it. If it has already been discussed please link me. If not.... I was working on a track. It was sounding great in stereo, great in my car, great when I showed it in stereo, but when I went to play it over a mono speaker system that is when I noticed a ton of masking and stuff was not cutting through the way it should have been. I subbed to Izotope and I was trying to figure out exactly how to remedy the issue im having. I just don't know what exactly I am supposed to do or look for in a plugin to fix this issue. Any other experienced engineers out there that can give a hand with this problem? Thanks in advance, sorry if it was posted and I overlooked, just would love to get this track done.