What shifters? What cassette? provided the shift cables aren't sticking and everything is routed smoothly, it sounds like there's a mismatch between shifter # and cog #.
Even though the manufacturers like to say "9 speed derailleur", it means nothing: derailleurs don't care how many cogs you have on your cassette, they simply respond to what the shifter tells them to do.
All modern derailleurs can shift basically anything you want. For example, I use a "9 speed derailleur" but with a 7 speed cassette and 7 speed shifters.