Sounds like your devices are offline. It could also happen if you have wiped your device. The devices need to be online for the “disenroll device” action to take effect on the device. Once this action is initiated, you can mark your devices as disenrolled and free up the license. For this, you need to go to Management > Devices, select the devices you wish to remove and click on “Mark as Disenrolled” option under Manage tab.