The iPad’s operating system (IOS) has a limit of 11 pages for apps. The first app you install whose icon can’t be represented inside those 11 pages, won’t appear at all and the only way it can be run, is to do a text search for it. It is an odd design decision, but it gets worse because even if you start creating “folders” and putting apps inside them, the “invisible” icons won’t automatically move and appear in the just freed space, as one might expect them to do.
One “solution” around the problem is simple: just shutdown the iPad and then turn it on again. On boot, IOS will populate any free space it finds with the previously “invisible” icons.