5 to 6 users not found in user search - Hybrid farm - SharePoint 2010 content - SharePoint 2013 Search

Hybrid farm SharePoint 2010 content crawled by SharePoint 2013 Search.

Everything working great. Out of 500 users, 5 to 6 users not not found in "User Search" of Enterprise Search page.

Profile incremental update has been done few times in SharePoint 2010 farm, no use.

Help appreciated.

Apriori algorithm [association rule]