XR500 Not Blocking some Devices

there are times I wish to block a device on the network and it's fairly straight forward to do, But for some reason, it doesn't seem to work out that way.. I'll go into the routers web page settings, I'll select Device manager from the left column, and pick a device that I want to block... For example, A tablet.  I'll click on it, and on the right side of the screen a pane will open showing the devices info, such as Mac address, IP Address, Connection Type and the name of device.. Well, I'll click on "Block" and then "Save".. The Icon turns black and voilla it should be blocked... Well, not all the ones I select actually get blocked.. It will still be connected and able to access the internet. Some devices that i select to block will have to wait for 2-3 min before the blocking kicks into effect, and other devices, it doesn't do anything... This also applies to some of the ether wired devices such as the livingroom computer that's connected VIA Ethernet cable.. .As I mentioned The devices that I'm blocking are turning black after clicking on save, but it doesn't seem to make any difference with certain devices.. And an added note... I'm making sure that it's the correct IP addresses of the devices since I can't block by Mac Address.. . Have no idea why... Any help would be appreciated...