I've been having some issues with all my mixes. Whenever I pan rhythm guitars and backing vocals, they just vannish when I hit the mono button on my interface or play it on a cell phone speaker (I know it's terrible sounding, but people use it...). I'm not doing anything unnusual during mixing, other than using Brainworks Hybrid to widen a little bit my master bus, but the same thing happens with it off. I have also tried using different amp sims on the rhythm tracks but it still didn't work. I know this happens even on commercial tracks, but they are usually not as terrible as what happens on my mixes. What should I do to prevent the effect from being so bad?