It should be pretty easy for them to find out at what bank the were deposited and then who did it.
Once you have that information I would contact the bank or if possible show up directly in the branch.
The check is payable thru:
First National Bank of Omaha
Omaha, NE 66102 and
First National Bank & Trust Co
Columbus NE 68601
I have this information from one of my checks. If you can find out the check numbers that were sent to you, you should be able to do something from there. Mail fraud also springs to mind so you may want to contact your Post Office and have them look into why your checks were not delivered.