How to fix it?
Make sure you mix your song with your mastering fx chain activated (so everything fall in place even when it's cranked as hell by the limiter). You can use some volume automation in the breakdown or you can decrease the gain of that particular sample clips in that region (if you bounced that particular midi tracks) until you hear it in a lower volume which will keep the initial idea of your production. And again, make sure you do all of this with the master fx chain activated.

 

A lot of people will mix too low, or mix in a way where the drums are buried or the guitars are buried or the vocals are buried, then try to compensate for all this by putting heavy compression, EQ and limiting on the mastering chain. Your mastering chain should be very minimal and never affect the sound of the mix itself, other than augmenting the volume and maybe adding a dB of sweetening in the upper and lower end. It's too easy to add 5 or 6 dB of extra volume with limiting or boosting the low end with EQ. Your mix should be able to stand on its own before the mastering chain, just at a slightly lower overall volume.

 

When i meant about mastering fx chain, do not think about tons on instances. He was talking about loudness, which actually influenced in a negative way some of his song's regions, so when i said "mastering fx chain" i meant only about a limiter instance.
Yes, you're right with everything you say about the "compansating from the master bus", which is the most wrong approach of fixing some of the mixing stage mistakes, but even mastering have it's own stage of work, sometimes you don't need any other instance at all, sometimes you do. Mastering is not about fixing mixing stage's mistakes, but if a client sends you a really bad mixdown and if you really know what you're doing, then you can add some tools on the master bus as well in order to fix some things, but not drastically, because it's impossible.

 

Yeah I use volume automation clips at the breakdown or break beat parts. But like I said, it only happens when I have to export a very loud song, as client's requests.
Normally I don't have much work to do in the mastering stage, cause I try to mix my projects pretty well instead :D

 

Absolutely. I think having a mastering chain (beyond light compression, EQ, etc.) gets out of hand when it's being used to overcompensate for bad mixing, but I'm talking about when the mixer does it, not the mastering engineer. If someone doesn't know how to mix, they certainly aren't going to know how to fix that mix after the fact. Probably a good 60 percent of a mastering engineer's job is to fix bad mixes.