Here's some addition stuff to try. You may have some phase issues with your overhead mics. If so, you're probably getting some frequency cancellation when you sum them to mono making them quieter. A visual inspection of the waveform and manual realignment of one track (or use of a delay plugin), or a plug-in like InPhase by Waves can help a lot in this situation to ensure they are in-phase with each other and sum to mono without issue. It could also be a polarity issue on one of the tracks. If this is the case, try flipping the phase on one side to see if that helps eliminate the issue.