Aside from having the toolbar showing (the keyboard shortcut is just “T”), there might also be at play 1) how many of the view modes the toolbar is set to show (use the arrow at the right end of the toolbar to display and select or remove options from the toolbar) working in concert with 2) how wide the left and panel columns are.
For example, on my 15” MacBook Pro, and with all the view modes set to “on” in the tool bar, when the left and right panel columns are set to their full width, the first view mode that disappears is the rightmost, that is, the thumbnail size slider.