Hey There !
This can be hard to tell you what you could improve if you don't tell more about it :
- Which DAW are you using ?
- What's your Mastering chain composed of ( plugins, order )
- Are you using in the box plugins from your DAW or 3thrd party plugin ?
- Is your Mastering Chain on your Master Channel, or do you use a Mix Bus & what are the levels ?
And I could go on... Mastering is very complex, but at least it will help to know your DAW and the Limiter you're using.
There are so many different ways to approach and deal with Mastering and a "Default" Mastering Chain would not be like making the magic happen all the time, because that will depends of the mix.
Before thinking about mastering & loudness gain, you have to take care of Gain Staging & EQuing in order to have a well balanced mix in terms of levels and frequencies, so you have to filter, EQ & fine tune each individual track and be happy with how it sounds when reaching a "Vanilla" Master Channel ( no audio processing at all ) with it's level set at 0 Dbfs and peaking your Master Fader at around -6 Db / -4Db (max) in order to have enough room to push your limiter.
Your master EQ should be used to fine tune your mix only, not to correct, that has to be done before at the mixing stage. Also, you would like to check your mix in Mono and check those high frequencies in terms of Mid/Sides levels.
But again, hard to guide you without at least knowing the DAW and Mastering Chain, and even how, there's no magic receipt that you can just apply all the time, it's mix & audio material depending.