The format of a 'Header Detail' import file would look something like this:
H, field, field,...
D, field, field,...
D, field, field,...
The format of a 'Detail only' import file woud look something like this:
H, field, field, D, field, field,...
H, field, field, D, field, field,...
I suspect that you are mis-matching the import file to the template type, and using a file layout in the latter case. Ryan's suggestion to compare the sample files is good, to ensure you are picking the correct template type.
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