The Post Office quite often doesn't scan mail sent to various IRS addresses as delivered.  Just too much of it, and they have better things to do.  Were there any checks in any of that mail?  Have they been cashed?  Did the clients prepare their own envelopes, or did you do them?  Are you sure the address was correct on all of them?  Do you have other clients who sent mail to the same address, which has been scanned as delivered?