What would you expect if you could get through on the phone?  "OK, wait a minute ... yes, I just found the check here in this drawer.  We'll cash it right away." ?  

How do you track a payment made with a cashier's check?  That's an even worse waste of money than certified mail.  I suppose for a fee the bank will tell you what happened with it.  

Extensions with payments are sent to a different address than those without.  The address is a private lockbox.  These were mailed?  Did you provide the envelope or address?  Did they write their name and SSN on the check?