trying to get some slack out of my chain cuz i went back up to 20" tires ( had 18" on with a 14 tooth )

so today i put my 20’s on and went back down to the stock 13 front sprocket

and i got both screws out of the carrier but i cant get my adjuster to move?

my 400ex i can do with a screw driver and a light tap.. but my 300ex is proving difficult .. any suggestions on safe ways to get this thing to adjust?