Perhaps they lost or misplaced the check and are afraid to ask for another.  Why not call them and tell them that the check has not come across you statement yet?  (In rare cases, someone can cash a check and it just gets lost in the system.  I had this happen once for a magazine subscription.  Magazine company swore they cashed it but it never cleared my bank or was taken out of my account.)  If not, ask your bank how long they will honor that check before they would reject it.