What dictates whether or not the value of a field will display correctly if I use the "First Value of Field" function in a summary header? I can pick the field I want from the list but there's no sample value in the column and so nothing appears if I choose that field to be included in the summary header. I've had this problem in the past and thought it was a case of making sure the field was included the summary but that doesn't seem to be working in this case.
I've found the problem - the first value of the field was empty! Looking at the table view, there was a line right at the start of the report that matched the detail template but gets filtered out because of its append template values. I hadn't looked at the unfiltered table view in a while and didn't spot this obvious cause!