Things to check that might be overlooked.
Bolt or bolts holding the replaceable hanger tight?
Cassette tight with 10spd spacer behind the cassette if we are talking 10spd.
Sometimes you might have to back the b-tension off all the way to get it to drop down fast enough to the small cog.
Skewer tight?
Put slack in cable and pull housing sections back and forth to confirm they are sliding smooth and free?
Last one I can think of is oil the bottom bracket cable guide, seems silly but that thing will mess with you.