Reading your post, I'm thinking 'arrangement' is the first thing to consider. Even if it's someone else's production, it may just have too much going on regardless of how you try to mix it (competing parts, a sense that they didn't really commit to anything so just sent lots of things). In that case, feel free to mute things which your ear tells you are just detracting from the song/vocal/groove whatever.
As Magnus says, you could, for example, take the vocal and work 'backwards' (if you're used to working from drums up) adding in essentials one by one. It might highlight what's not necessary to the mix or is simply 'padding' to add low in the mix.
If it's instruments/vocals competing for space in terms of frequencies, then it could be something like too much mixing in solo? Making something sound great and then another thing sound great but when combined you've added in similar frequency moves and the more dense the mix the less separated parts now are?
Referencing commercial tracks in the same genre frequently while mixing (Warren's mixes come with the multitracks) is key and having a vision for what you want to occupy what (what's going to take our focus... be brighter... be drier... etc.) before ever touching a plugin will hopefully help.
Hope that helps in some way at least.
Best wishes with your mixing,