May be the IO forgot to do it there, Just find the I-94 online and you will find the I-94 expiration date. Another choice is go to the local CBP Deferred Inspection Office to solve this issue.
Find the local CBP office here (click the Deferred Inspection Sites on the up right side to open the link):
http://www.cbp.gov/xp/cgov/toolbox/contacts/deferred_inspection/overview_deferred_inspection.xml
Find the local CBP office here (click the Deferred Inspection Sites on the up right side to open the link):
http://www.cbp.gov/xp/cgov/toolbox/contacts/deferred_inspection/overview_deferred_inspection.xml