Index Utilization Issue

Hi,

I have a query of format

{ 
   $or: [
        {  
              {.....},
              $and: [
                      {....},
                       {....},
                      {  
                            tags: {
                              $elemMatch: {
                                $or: [
                                  {
                                    tag_id:     "*****",
                                  },
                                ],
                              },
                          },
                       }
                  ]
        },
        { similar as above }
    ],
}

This query is not utilizing any index although present. There are 2 clauses in $or. When I remove any one of them index is used. Why is this case? I tried removing tag filter above and index is used even when there are 2 or more clauses.

Thanks for help