I'm checking my mixes in mono, but then I notice that the cymbals are less loud than in stereo. Is there something I can do to minimize this difference?
I'll take it that your talking about acoustic drums.
You could try bringing the overheads in a little. I usually have overheads and rooms panned between 60% to 80% and very rarely hard left and right. Or you can try using EQ in mid/side and boost some higher frequencies in the middle. Maybe a combination of both.
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.
I've got them panned hard left and right, indeed, because I bussed them and in Mixbus the channels are connected to either the left or the right input and then the channels dont respond to panning anymore. But I'll setup another mixer construction to be able to pan and see what it does.
I placed the OHs in phase with the snare drum, this always works fine, but I'll check if I can improve the phasing a little.