There are any number of reasons why devices that you expect to see might not show up in your scan results all of the time. The most common reason is that the devices in question are actually on a different subnet from the machine doing the scanning, such that other subnet must first be added as an additional scan range. Sometimes, routers that function as network extenders also “share” a MAC address and are not distinguishable from each other in a scan.
Here’s our basic FAQ on this: http://10base-t.com/faq-and-help/ip-scanner-faq/#ezfaq_5
But also make sure in the IP Scanner prefs that you have 'show hidden' turned on and possibly also ‘include non-local devices’.
Finally, if nothing is working and you are certain that those devices are part of the same subnet that your computer is on, try pinging them. If you can ping them and they still do not show up in the scan results then there is something seriously wrong and you might want to reset the IP Scanner prefs as something could have gotten corrupted. Here’s how to do that: http://10base-t.com/faq-and-help/ip-scanner-faq/#ezfaq_3
