First of all, listen to the bass of the song. is it muddy? Does it feel like there is too much going on? This may be the issue, as often is the case. Check what instruments you have and if they are introducing noise to the mix. A quick fix is to high pass everything, e.g cut out the bass frequencies, just below the lowest note. The chart below shows that the cymbal’s range (green) goes down to 200Hz. by cutting every frequency below 195Hz, you have the potential to give a lot more room to the bass instruments. Reverb is a bad effect for allowing mud to reign supreme in the mix. Remember to high pass the bass on the instruments’ reverbs as well as the mud there will be worse. If that doesn’t work, try dipping the EQ around 250Hz with a narrow Q notch. If the bass is still too much, especially in combination with the kick, try side-chain compressing it.

Does the mix have a piercing sound? This is where the physics of the human ear comes into play. The 2-5kHz range is the range of particular resonance in the human ear. It is an essential range for the clarity of speech but often if it is too high in the mix it can be fatiguing to the ears. 4.5kHz, in particular, is a nasty frequency and rarely benefits from being boosted due to our sensitivity to it.

Do the combination of instruments clash at certain frequencies? Instruments have extended frequency ranges beyond their notes, they have harmonics. You will never get an instrument to completely occupy its own space but trying does help. What I mean can be described using an acoustic guitar and vocalist. Often the chords in a guitar will contain the notes and frequencies that the singer is occupying, this is not a problem since the guitar also plays the lows and highs which the singer cannot reach. Due to this, you can look at the main frequencies the singer uses and dip them in the guitar’s EQ. The singer will be much more prominent without the guitar’s range being compromised.

If your mix sounds a bit flat, it may be because there are no dynamics left after you have compressed it. Try halving the ratios of each compressor and playing the track. If it has more movement and life you know what the issue is. (by halving the ratio you can easily go back to how it was by doubling it – a matter of convenience more one that will fix it.

Does it still sound bad? Perhaps you haven’t used enough spacing. By panning and pushing instruments back in the mix. This gives the main vocals and instruments more “forward” space. This can be done by using Reverbs and rolling the bass off to make thins sound further away, you can make them less focused with a chorus effect. Have instruments panned from side to side. Consider how they would be on stage and how a drum kit is laid out to keep it natural. Remember that no instrument would only be in one ear so avoid the 100% pan unless it is deliberately needed.