You certainly can sum a bound detail control inside of a header so there's that. Can't recall if you can do that with an unbound detail section control.
It would help to know what the data and expressions look like and what the expected result is.
Perhaps you can solve this with a hidden textbox in the detail section and set the running sum property, then in the header, refer to the hidden control.
EDIT - forgot to mention that perhaps your problem is that the header control is trying to sum an unbound calculated control in the detail section?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.