You press the mono button and the mix gets smaller. Not just narrower, which is expected, but thinner and quieter, as though somebody turned parts of it down. Something is cancelling, and there are only a few things it is likely to be.
Narrower is expected, thinner is not
Worth separating these before you start hunting. A mix in mono should sound narrower. It comes from one point instead of spreading between two speakers, and that is simply what mono is. If it sounds narrower and otherwise the same, nothing is wrong.
What you are looking for is a change in weight or level. The bass loses its body. The vocal steps back. The whole mix drops by a couple of decibels the moment you press the button. That is cancellation, and it means part of the sound exists only as a difference between the channels.
The four usual causes
A widener on the master or a bus
Most of them work by increasing the side signal, which is the part mono throws away. Bypass every widener and press mono again. If the drop goes away, you have found it.
Delay-based width on a single source
Copying a mono source to the other side and delaying it by a few milliseconds sounds wide and comb filters when summed, so the tone changes as well as the width. Common on synths, guitars and backing vocals.
Bass with stereo information in it
A wide sub, a stereo bass patch, or a widener that reaches below 150 Hz. This is the one that costs the most, because low frequencies carry the weight and clubs play them through a mono subwoofer.
Polarity on multi-mic sources
Two microphones on one cabinet or one drum arrive at different times. In stereo it reads as depth. Summed, part of the low mids cancels and the source loses its body.
Finding which one you have
- Switch to mono and leave it there. It is easier to hear what is missing than to catch what changes.
- Solo the low end first, below about 200 Hz. If that is where the weight went, you have a bass or widener problem and you can stop looking anywhere else.
- Bypass every widening plugin in the session at once. If mono stops hurting, reintroduce them one at a time.
- Mute the wide elements one by one and watch the master level. The one whose muting makes the mix get louder in mono is the one cancelling.
- On any source recorded with more than one microphone, flip the polarity on one of them and listen for the low mids coming back.
What each one needs
A widener wants turning down, or replacing with width that came from a real performance. Delay-based width wants a shorter delay or none at all. Polarity wants flipping and checking. Bass wants making mono below a crossover, which keeps everything above it exactly as wide as it was.
There is also a fifth option, which is to keep the width and move what would be lost. Energy that only exists in the side can be summed into the centre, so it survives the collapse and the stereo picture stays where it was.