Unable to change the pacing / move a keyframe of a motion tween animation

As the title itself implies, I learned that the correct method of moving a keyframe within a motion tween layer is to click on the keyframe per se, then a "tiny box" icon will appear next to your mouse pointer, indicating that you can click and drag the keyframe to a new frame/time. However, in my several attempts, I consistently came to no avail. I even scrutinized Adobe Animate's handbook and it explains exactly the same steps I did. By any chance is my program partially out of working order or something? Doesn't seem to affect other Animate users as I rummaged through the Web though. Consider assistance needed for this issue.
Thank you more than words could express, in advance!