Widgets of installed apps do not appear in list
This can (and certainly will) happen when installing apps on SDCard. Normally, the system should prevent this (or rather the dev, by defining the "install-location" accordingly in the app's Manifest).

So if the apps were installed on SDCard, go to Settings→Apps, scroll to the apps' entries, open them (one by one), and move the app to internal storage. If the widgets do not appear right away after that, they should do so at least after a reboot.