A Survey Target record is submitted without Question Response records.
This occurs when the user is missing the Survey_Question_vod__c records related to the Survey Offline, and none of the Survey_Question_vod__c records are marked as required. The most common reason for the Survey_Question_vod__c records to be missing is a timing issue between the time when the user syncs and when the records (and related records) are last updated.
Be sure there is an active Vmobile Object Configuration for the Survey_Question_vod__c object. Be sure that Enhanced Sync is turned on for the Survey_Question_vod__c Vmobile Object Configuration. This ensures that the records, which the user has access to, sync offline.
CRM Help Documentation: Survey Overview