Missourians who have not received their advanced federal tax refunds are being urged to contact the Internal Revenue Service within a month. Those are the $300 and $600 checks that were mailed out in the summer and early fall. IRS spokesman Janice Lawrence says about 5,500 checks have been returned to the agency as “undeliverable”. Their average value is $313. Lawrence says Missourians waiting for those checks should contact her office by December 5th.