Lightroom will only show you the images you have in the opened catalog.
As you can only open a single catalog at a time and thus can see catalog specific images.
Put them all in one catalog.
Basically you should only have one catalog and all images go into that one catalog. You divide, separate, them by using Keywords and or Collections and how you set up your folder structure on the drive you place your images. I personally do that by year, Pictures 20XX, then by the date the images were taken 01-02 and so on.
Lightroom is not a typical file browsing program. In order for Lightroom to know where your images are located you must go through the import process. Importing the images does not consume them into the Lightroom program. It is simply a process of telling Lightroom where the images are located. Because of this unique approach, you cannot simply see all of your images. You can only see those which have been imported. That is why it is best to work with a single catalog.