pppoe not refresh

Good evening

I have in my network 5 NAS and a radius server for authentication. The problem encountered is when a client authenticates and connects to the network normally but when it disconnects the NAS does not refresh at the interface and always reports the online account. As soon as the client wants to reconnect, authentication will fail. You will have to manually remove the interface pppoe in the interfaces window so that the client connects again.
How to fix problem?
ps: The customer uses an RB750
thank you

There is no one to help me???