a 10 sp. cassette has the SAME STACK HEIGHT as a 9 sp. Cassette.. what is happening is the Derailleur cage is not able to freely push the chain toward the Low gear position because it's now hitting the larger gear's face.
you need to install a hanger extension (AKA: Wolf Tooth) and check your cable for any slack when it's in the Smallest, high gear position, then adjust that cable tension to eliminate any slack whatsoever... the derailleur should move as soon as the shifter is moved from the "High Gear" position...
quit thinking the shifter is the issue... look at what was CHANGED (low gear cog diameter/tooth count) and it's relationship to the DERAILLEUR.
Last edited by maddog34; 05-20-23 at 11:18 AM.