Reporting Services: Tablix RepeatColumnHeaders doesn’t work on some reports

So I've got various reports that consist of a DataSet rendered in a Tablix… pretty garden variety stuff. There is a property for the Tablix control named RepeatColumnHeaders, which I have set to True for each report in question. The explanation for this property states “Indicates whether column headers are repeated on each page on which part of the data region appear.” Sounds pretty straightforward, but on some reports it works and on others it does not. I can't seem to find what's different about the reports that might affect this. On one report where the headers do repeat, there is some fairly arcane grouping mojo, but in an example where it doesn't work the Tablix only has one level–no grouping. I would expect the multi-nested one to be the problem, not the flat one.

Maybe it's a different problem altogether. I threw together a simple Tablix rendering SELECT * FROM Foo, accepted all the default values, which results in RepeatColumnHeaders being set to False, and lo and behold the column headers do repeat for that report… Grrr.

Any insights greatly appreciated.