It hides itself below 1100px of window width, where there is no room beside the text. On a narrower window, use the bar pinned to the bottom instead.
On a wider window it also steps aside while anything wide passes underneath it, such as a product gallery or a full-width image, and comes back once that has scrolled past. Some themes, Twenty Twenty-Five among them, take product pages and wide images close to the edge of the window, so there the rail is often out of the way. That is on purpose: a rail on top of the product photo is worse than no rail. The pinned bar has no such problem.
Still stuck? Ask the developers who wrote it. We aim to reply within one business day.
