[suicidaln00b]

haven't touched a DAW in like two years tbh, but in my experience that empty feeling can be fixed by (1) filling each frequency range with instruments and/or (2) using Maximus

mid-range instruments often contribute most to that feeling of "fullness" so this is usually where the bulk of the sounds are; so stuff like the melody, chord progressions, snare drums, literally any mid-range instrument that you can think of
lows are pretty important as the song may sound oddly hollow without them; so stuff like kick drums and any other bassy instruments that you can think of. generally speaking there's less instrumental variety in this range and if you put too much here it'll sound muddy as fuck
and for lack of a better explanation highs add a noticeable sharpness/crispness to mid-range instruments. also some examples of instruments that would appear here would be like hi-hats and cymbals

also you just need to get gud at harmony/chord progressions I guess, otherwise you'll just be throwing instruments everywhere and it'll sound like piss
learning this is pretty nontrivial though, and is something that just comes with time

anyway if you're using FL Studio I usually just vomit the Maximus plugin onto the master track and the use the preset "Max loudness 2"
it's lazy as fuck but it just werks, but I would like to note that this isn't any substitute for actual good mixing