Unlike derailleur bikes, internal gear hubs shift best with no tension on the chain. Either stop pedaling while shifting or at least ease off on the pressure.

Have you been feeding it oil through the oil port? The first thing I'd try is squirt a couple ccs of oil into the hub and ride it around a bit to spread out the oil. Also check the cable and shift lever: if they cannot move freely shifting will suffer. Lubricate or replace as needed.

Your description does not strike me as indicating an adjustment problem, but having changed the tension it may now be out of adjustment. The bell crank has a round window with a slot in it. Place the hub in 2nd gear and adjust the tension so the "N" in a circle appears in the round window and the line aligns with the slot: