A few drops of penitrating oil is what I would try first. I have
three of those, if crud works it's way inside it can "screw" them up.

If your's is the same as mine, on the side of the selector (the right or left selector) there will be a small section with a cutout with two prongs sticking out, those are the ends of a circle clip, squeeze them together and it will come apart but do it over a big bench covered with a towel, tiny parts can fall out, springs, a pair of tiny ball bearings (if I remember right?) and a ratchet pawl. I wouldn't take it apart unless it's the last resort, I do remember it's a pain to get it all back together, getting that clip to fall in the slot all the way around was a bugger on top of keeping the tiny parts in place. Flag down a Snap-On truck and hand it to them, let them put new parts in it for you for free.

Or if you know a tech, leave it with them and let them give it to the Snap-On guy, I'm sure they still come in the shops every week to pickup their payments.