https://www.reddit.com/r/typewriters/comments/1mp43lf/a_sticky_shift_issue_olympia_sm9/
Over time, dust and oil can clog up in the bearing and along the track that the segment glides up and down on causing this sort of issue. You can flush it out with mineral spirits (or something similar) while you actuate the shift key to remedy the issue. When you're done, then add back a drop of light (sewing) machine oil.
While you're at it, most basket shift machines load them with large springs to assist with the weight and to help the return. They're typically attached to metal brackets that can be easily formed with pliers to either increase or decrease the force as necessary to suit your touch and preferred response. (YouTube videos may help here if necessary.)
If that doesn't remedy it, then take the hood off and look closely at the mechanism to see if something is physically impeding it.