Amit,
For Sales Return you should use 05 Inspection type as it is a stock relevant 06 is Non Stock Relevant and Where you can not take any UD only you can do Results Recording.
May be you need to change the Inspection Plan Usage to 05 and Reassign the plan to existing Inspection lot to do Results Recording.
For Sales Return Stock Relevant Inspection Process Please refer my reply from below thread http://scn.sap.com/thread/3620881
BR
KK