Mix loudness is achieved by reducing dynamic range
In general, the loudness of a mix is determined by the difference between the loudest element (or transients) in the mix and the average level of the mix.
In Practical Terms
If your mix is not loud enough, there is an element or transients (spikes) in your audio that is too loud relative to the rest of your track. Reduce the level of these elements or reduce the level of transients (via compression) to increase the average level of your mix.
Illustration of a mix where the average track level (green) is quiet relative to 4 louder elements / spikes (red)
This track has been made louder by reducing the level of the 4 offending elements / spikes. The overall average mix level is now louder (after normalisation).
