Welcome to Android Central! I assume these files are images, correct? What file format are they in? Use My Files or any other file manager app to look in the directory where those image files are located -- is there a file called ".nomedia" as well? If so, delete that file and see if the images show up. ".nomedia" instructs the system to ignore that directory (and any subdirectory) when looking for images to add to the gallery database. Be aware that WhatsApp might add ".nomedia" back after you delete it, in which case you should copy those image files to a different directory (like /Pictures or /DCIM/Camera).