Try these things when your mix just isn't working. Sometimes we don't know what we don't know.
But these few things can make almost any mix better pretty quickly. These are meant to be introductions to topics especially if you're new to mixing. but these few things can certainly help any mix.
Filtering, Space, and Saturation the main things can change how your mix feels, hits, and vibes.
Filtering - using high pass and low pass filters to unmask, declutter, and make room for other mix elements.
Space- using things like panning, reverbs, or delays, to either push things back or pull them forward in a mix. Chorus or modulation comes into play here. Anything to manipulate how mix elements sit spatially.
Saturation - This can be a rabbit hole for sure. At its core... saturation is turning up the temperature on your mix. Most affordable interfaces are sonically transparent... BORING. We need saturation to add color and excitement to an otherwise BLAND sound.