It is usually the recording itself.

By the book, the Recording Engineer should do "Mono Mixdowns" to check for poor sound. Essentially, if there are large phase differences between the Left and Right channels, they won't sound good when combined.

"3. MONO IS THE KEY
How can you assess if you’re track will sound good in the club?? The answer is by testing it in Mono. This is yet another step where you have to be critical of your stereo imaging and what you’re using to give your sounds the width! Some plugins will make your stuff sound super wide and good in stereo putting a big fat smile on your face, but when collapsed to mono, all you’ll have is a weird look due to all the phasing differences that you’ll hear.

- If you’re on Ableton, slap a Utility on you master, pull the width all the way to 0 and assign a hotkey to it. So the next time you’re making a sound or mixing stuff down, keep pressing that hotkey every few minutes to see how the mix is sounding in mono! (I’m sure there will be some way to replicate this on other DAWs as well)"

Live music can suffer from the same problem if there are big differences in the distance of the L & R mics to the performer(s).

Some producers put these widened plugins or use techniques like the haas effect to widen certain sounds, but they tend to sound like shit - or even completely cancel out certain sounds - when the tracks are summed to mono.

I play at a venue that provides only one XLR lead. Even if its 2 XLR i think most venues will still sum it up along the chain.

I just play in mono.

I do notice some tracks suffer but it aint my fault.