Which surfaces are intended to set the bracket position, and which merely support it? Mark those on the drawing first. A clamp should hold a located part, not force several conflicting contacts to agree.
Then ask the designer to explain each contact against the part drawing. Compare the bench samples with that scheme before changing the clamp. Their variation may show which contact is interfering with seating.
Has the clamp supplier actually seen a rocking sample, or have they only been told it needs holding down harder? I'm no fixture designer, but buying a bigger clamp seems a surprisingly expensive way to postpone that question.