A lot of times it's a matter of making room for them in the mix. If the vocals are trying to occupy a space that is already full, they'll get burried. Space, in this case is comprised of the following elements;
Pan - vocals usually sit in the center of the soundstage. If there's a bunch of stuff already there, it's too crowded for them.
Frequency distribution - The vocals will have most of their energy in a certain frequency band. If that frequency band is also very busy, it's too crowded for them.
Time - If there's too much happening in the araingement during the vocals, there's no room for them.
Volume - If other things are too loud, using up the overall dynamic range, there's no room for the vocals.
These elements all interact to create or prevent the vocal from sitting correctly. It's Ok if the Bass is panned center and playing at the same time as the vocal, because it's in a different frequency range. Or some other instrument may be in that freq band, and playing at the same time as the voc, but it's panned out of the way...that's ok. It's when all the elements line up that things compete.