Had this issue just yesterday except the other way around. Went down a gear and lever wouldn't come back up! An absolute pain on roundabouts. Got home, sprayed half a can of degreaser around the area, lever pivot and the actual gear change pivot closer to the engine. Then WD40 around the pivots.
It was a case of:
Does it move - Nope.
Should it move - Yep
All good now!
2002 Honda CG125 > 2008 Triumph Bonneville T100 > 2007 Kawasaki Versys