Why are all my spaces are discovered, but not showing up on the main screen?

After the units are discovered, you need to assign a screen location on the display for the room. 

The MRMS can display 8 rooms. If you have more than 8 SRPM or 4 SRCM (each SRCM will show up with each of its 2 rooms as a location) on the trunk, the MRMS will discover all of them. The user assigns which ones to display, up to 8, in what location on the MRMS screen.