I have one alternative solution for this
1. set sort order for measure column (I'm Applying Descending order)
2. create another measure column like BOTTOMN(RSUM(Measurecolumn),5)
3.if you are using Ascending order then use TOPN instead of BottomN
You must have a set of dimension attribute which makes each row unique in order for your ranking to work. As you mentioned, your report appears to be returning the same values for several rows so you'll need to distinguish each row.
If you could send across a screenshot then you might get some specific recommendations about what your next steps should be.
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