Ibrahim,
As you know some devices may not show up as connected to others due to their CDP neighbor adjacency not being determined for various reasons.
Please look under unconnected devices view on Topology to see if your "missing" devices are there. Another place you can check the same thing is within Device Center - click on the "Topology" tab near the top of the page and you should see the topology specific to that device. For something like and ASA (which doesn't support CDP), it will always show as "alone" and always be put into the Unconnected Devices view.
