I'd be looking at removing the centre trim so you can get nice and close to the mechanism itself so you can visually inspect it to see if anything seems out of place. If you can't see anything broken or getting in the way, you can try to spray some lithium grease into the moving parts of the joint. Depending on your available time, if it were me, I'd be taking the whole shifter assembly out and dismantling it, then cleaning it, then lubing it up and putting it back in. Sometimes visually inspecting things don't reveal any broken bits, but when you take it apart, if something falls off that shouldn't, then you can usually identify the problem.