PickList not updating during LEADs Conversion

PickList not updating during LEADs Conversion

I have a custom field (2 value pick-list) in leads which needs to transfer into Contacts and Potentials during conversion (Convert Button). similar pick lists are available in both Contacts and Potentials. it works for Potentials as expected but not for contacts. i also did Lead conversion mapping correctly for correct layouts. the pick list value in Contacts is always empty. however, other fields get transferred with correct values but not that pick-list value. what cause this problem?