It sounds like a book you have copied to the Touch is corrupt in some way and the Touch cannot process
What happens when you sideload a book to a Kobo device , a file is put somewhere on the devices drive. When the device is disconnected, it checks for any new files. If these are files of the type it can display, it processes the files and adds details about them to an internal database. The device uses this database to display the library lists.
If there is a problem with a book during the processing, it does not get added to the database. But, the device will also stop processing other books. So, if you put 10 books on the device, and the first one is corrupt, then none of them will show in the devices library. And as long as that book is on the device, no other sideloaded book will be processed into the devices library.
If you have only put one new book onto the device, you know it is bad. But if you put a lot on together, or still have some from earlier that have not been processed, you have to go through them by trial and error until you find the one causing the problem. The only thing you can do is go through the files on the device when it is connected to the PC and remove those that are not shown in the library on the device. Then put them back on the device one at a time, disconnect the device and see if it is added to the library. If it isn't you have the bad book and can probably add the others safely. But, do those a few at a time in case there are more problems.