Computer not registering on my Xfi app

Hi!

I have a whole bunch of devices that show up on my Xfi app when they are active, however, there are a few that don't (namely my son's computer).  I can't seem to get his device to register, in turn, I can't set it up in a profile for downtime.  This wasn't a problem on the older Comcastd gateway we had, only this new one we got just a few months ago.  We are running a Comcast gateway, so it's not third party.  How do I fix this issue so that all devices online in the house register on the app?  Thanks!