The first thing I would check is that the filenames don't contain an icon ID and that they use the ".STY" file extension.
A lot of Yamaha style files and voice files contain a special 4-character icon ID that's preceded by a period, and on higher-end models the icon ID tells the keyboard what image or icon it should display for the style or voice. But some Yamaha models don't display icons for styles and voices, and if they see a period in the filename they will assume that everything after the period is the file extension. If a style file has one of these icon IDs in its filename, rename the file and remove both the extra period and the 4-character icon ID from the filename.
If the style files you're trying to use don't have icon IDs in them, then make sure they use the ".STY" file extension. Yamaha uses a number of different file extensions for their style files, but the different file extensions are merely to help categorize the styles by indicating which types of style they are. However, PSR-E models won't recognize that a style file is indeed a style file unless it has the ".STY" file extension; a style file with any of the other file extensions will be ignored and the keyboard won't even display their names when you're trying to load styles. If that's the case, all you need to do is rename the style files to give them the ".STY" file extension.
Example 1: You put a style file named "Pop Ballad.S733.STY" on your USB flash drive but the PSR-E453 won't display it so you can load it. ".S733" is an icon ID, but the PSR-E453 can't use icon IDs so it thinks the file extension is ".S733.STY" which it doesn't understand. Renaming the file to just "Pop Ballad.STY" will let the PSR-E453 recognize that the file is a style file. Make sure you don't leave the extra period in the filename, because if the PSR-E453 sees "Pop Ballad..STY" (with two periods) it won't recognize the file extension correctly.
Example 2: You put a style file named "Pop Ballad.PRS" on your USB flash drive but the PSR-E453 won't display it. Although ".PRS" is a valid Yamaha file extension which is recognized by some models, the PSR-E453 doesn't recognize it, so it doesn't understand that the file is a style file. Renaming the file to "Pop Ballad.STY" will let the PSR-E453 recognize that the style is a style file.
There might be other issues with the style files that will need to be resolved before they'll play correctly on the PSR-E453, but if they won't display at all then the most likely reasons are the presence of an icon ID or an unrecognized file extension, so those are the two things I'd check first if I were you.