Hi, jptiger.
Let me just start from the main thing, that the charts is the place where everyone is competing against each other. So the main reason why your bands are not reaching the better heights in the charts is that other players keep getting better bands and keep promoting their records.
The suggestion I could give to You is to try to give for bands more instructions to promote. While you do promotion action, both currently charting records and a band itself become more famous. Another suggestion would be to try to hire better musicians to your bands.
Answering to Your question about changes to the game in last month - no, there weren't any changes how songs attain their level and fame. But You are right about poor state of single musician bands. Currently the game engine values the sheer power of band musicians' level. So those bands with single musician have constantly decreasing chances in the charts.