tempering your effects for a given perspective is critical in understanding this problem.
If you have a nice full sound with lots of stereo content (ie effects and processing on those particular effects), i wouldn't necessarily expect a whole lot of translation to a new perspective to be as accurate as it was with the initial conditions they are based upon.
Simply being, you changed your reference point to mono. and you have a ton of awesome things in stereo.....
That's like talking about a dank ass apple pie to some one who may not necessarily like apples, pie, or apple pie.
One the other hand, you might not have fully let your ears adjust to the change of view.
Another little tag is that, you also might be fooling yourself with some form of expectation that isn't there. I'd liken this phenomena to drinking vodka, when you think it's water.....
It could be a balancing issue, but i don't think that's the issue here...
aside from that, it'd be wisest to check your mix in mono from time to time. But you need to approach it with an attitude and understanding of what you want things to be (or sound, exist,etc) in that particular reference point.
It's an opportunity cost, you're experiencing.
That's a more philosophical answer and for pondering....
Do a quick google search as to understanding the science of your problem
"mix sounds good stereo but bad mono"
collect information, analyze, compare, conclude.
