Any network scanner can only detect active devices. If one or more of your devices has not accessed the wifi network in the past half hour or so it will not appear on network scans. Many modern ip devices (including iPhones and iPads) shut down their network connections when there is no data being transferred to preserve battery power.
Try this: access a web page on the iPhone/iPad that is not showing up in your scan - make sure it is using the wifi network and not the cell data network! Then run IP Scanner afterwards. It should show up, although it may appear as 'unidentified device' if the iPad does not broadcast it's specific identify.
Also, make sure in the prefs that you have 'show hidden' turned on.
http://10base-t.com/faq-and-he.....q/#ezfaq_5
