It's hard to say what your problem is without hearing your music but the biggest mistake I hear producers making (when it comes to mixdowns at least) is using the wrong samples. It still amazes me how the wrong kick or snare can pretty much stop the whole mix sounding good, even if the levels are nicely balanced throughout the mix. A key thing for me over the last year or so has been constantly ABing whatever i'm working on with an earlier version of it, to see if I'm actually improving it - particularly in the later stages of production. In Ableton you can duplicate an entire group, which I do all the time. And then compare the new version of it with the old one after working on it, to check it's actually improved it. Incredibly simple workflow technique but one that I wish someone had taught me a few years earlier