hello @azielenski , I would verify firstly if the device that has completed the BYOD process is still registered in the ISE side and that has not been purged, the device should be within the endpoint identity group "RegisteredDevices" if using the default configuration, next I would check the certificates If ISE issues the certificate you will see if the certificate has some problems in Administration>System>Certificates>Certificate Authority > Issued certificate .
In any case you can review the spw.log in the supplicant , this might give you further insight of what's happening with those devices , that log is allocated within the supplicant in /sdcards/downloads/spw.log
Let me know if that helped you.