During the mix process, do you switch over to mono and address issues? A mix can sound great in stereo, then you switch to mono and you want to cry. But you have to gird your loins and confront your instruments all piled up on top of each other, like they tend to get in the car.
What I do guarantee is it has absolutely nothing to do with your rendering. Most likely it's a case of your mix sounding good in the "sweet spot", and bad elsewhere.