Table.Distinct taking out unique rows

Hi there,

I have a dimension table and i've created a concatenation of two fields to create a unique id. I do a table.distinct against this, and it's taking out rows which are in fact distinct. When I filter the table to a particular set of data which I know it's removing, then do the distinct afterwards, it keeps them. If I do the distinct before the filter, they're all gone. I've done a Table.Buffer in the Table.Distinct and it doesn't help. I have no idea why these rows are being removed - any ideas?