This may be because these devices are inactive. The devices have to be active for the disenrollment process to be successful on the devices.
As the device is inactive, please click on the desired device and you will be navigated to the device summary page. Click on the Actions button to choose the ‘Mark as disenrolled’ option. The device will now be probably removed from the Manage tab list.
The delete option in the Actions drop-down can be used only for removing the pre-approved devices that have not yet been enrolled by the user.
To remove other devices that are enrolled in the portal, you will have to disenroll them:
Navigate to Manage-> Devices and select the desired device.
Click on the Actions button and choose Disenroll Device.
Confirm the action, enter the password and click GO.