I think i found the root cause here. The query which we were using brings top 15 records in asc order based on a column which has NULL or 0 0r 1. This was also one of the main columns in the index which did allow nulls, however if i change the index and not include this column then we the null column data would not be in the top 15.

Bottom line is that it was data issue, however i would still like to know from other folks that should i include my column which allows NULL values in my index or it just depends on business reason?

Thanks for your help.