If you see some devices and others not, yet are confident that all devices are stored in the inventory, it might have the following reasons:
Your access rights don't allow for other devices
In order to remedy the situation, ask your administrator to change the responsibilities in your user account.
You have applied a column filter
Click on "Reset Filters" and see if this remedies the situation.
You have deliberately limited the output scope
The scope pane in the upper part of the inventory window allows you to limit the output to certain locations, networks and so on. In order to reset any of these settings, click on the Field Label. For example, if you want to reset the location scope, click on "Location".
The scope panel also allows you to limit output to specific device lifecycle stages such as "planned" or "decommissioned". Check all "Stage" boxes to see if the device you are looking for will appear.