I can see what the problem is; it is down to a join problem between your data sources. Your sheet 3 is joining the ChannelData and Visit Lookup data sources on the Visit Type field (Data/Relationships) - this is doing effectively an inner join. If you look in your channeldata source you will see that there is only Visit Type data for 'New' and 'Total' - there isn't a value for 'Returns' so when it joins to the data with 'New', 'Returns' and 'Total' (in the Visit Lookup source) the returns data doesn't join. This means that your Case statement [Visit Revenue] never equals 'Return'
You can see this happening if you add the [Visit Type] field to Sheet 1 or the [channeldata].[Visit Type] to sheet 3.
One solution I can think of is using Data/Tables to join the sources (then you can specify an outer join) instead of Data/Relationships, or in this instance break the relationship between the ChannelData and VisitLookup sources and you get the result. The danger in this approach is that you need to ensure no other [Visit Type] data can makes its way into this source and get your numbers wrong.
I've attached my version.
Hope this helps
channeldemand-v1.twbx 34.3 KB