Lubricate the linkange and shafts with some light oil or WD-40.
Check to make sure there is some clearance between the linkage that opens the secondaries and the arm that points straight up where the link attaches to the secondary linkage. There must be some clearance between the link and the linkage where they meet, directly above the secondary throttle shaft, or it will be held open slightly.
If none of this helps, remove the baseplate and loosen all of the secondary throttle shaft screws about 1/4 turn. Gently tap the throttle plates to fully seat them, and re-position the shaft back and forth until the screws are centered in the throttle plate holes. Gently tighten the screws and work the linkage a few times, hold the throttle body up to a light, and make sure the plates are returning to the fully seated position each time......Cliff