The ratchet mechanism at the hinge of the parking brake handle is slipping. Either the button is partially depressed (stuck in the grip end and not coming up all the way?) and the engagement tooth isn't into the sprocket deep enough or the teeth are rounded off.
You'll have to take off the center console to look at it. That's only a few screws, but a little awkward to get the parking brake boot over the handle. The top of the parking brake boot has a rubber elastic in it, so you can be a little more "agressive" than you think.
