Exchange 2013 not showing the mailbox in "connect a mailbox" after mount and dismount

Dear Sir,

We are currently running Exchange 2013. We had a user who left our company, therefore, we will disable his/her mailbox by ECP. He/she re-enter the company, we mount his/her mailbox to the AD account. He/she left again, therefore, IT support disable his/her mailbox again. But this time have difference, we cannot see his/her mailbox again on the "Connect a mailbox".

Any expert knows where's the mailbox going? And why  this happens?

Thank you very much.

With regards,
Wataw