if they say they mailed it out the issue may be with the USPS. they do lose mail and sometimes its stolen or actually tossed in the garbage by PO employees. last year I mailed some payments from a dropbox within a USPS office with proper postage and a return address. more than 6 months later the recipient says they never got it and I know the checks were never cashed.
there have been newscasts on public TV about this including photos of PO employees dumping mail and other unidentified people raiding PO boxes using master PO box keys. The USPS has been unable to explain how non-employees could obtain the keys but I think it's very easy if there is just one dishonest PO delivery person. just fabricate a copies of the master keys.
your first step should be to try to contact the IRS to see if the check has been cashed. this may require repeated tries by you to get through to an agent. good luck.
