Please check following things,
1) Check "inspection with task list" and "Automatic assignment" check box is checked in the inspection type 06 assigned in the material master
2)Check the usage in the inspection lot created and usage in your Quality plan
3) Check if there are more than one valid inspection plans in the system
4) If you have usage "5" in Q plan and the same usage is in Q Lot , please check "prefered" checkbox for inspection type 06 in the material master and post customer return again and check
Thanks
DD