Seems that between the OP's statements on what he did and our statements on the film characteristics and I agree that with both Ilford and Kodak films we have very tough emulsions, we have ruled out most of the likely causes.

Uncertainty is a nasty companion but only the OP can remove him by developing the same film again whilst ensuring that the whole process from loading the film to processing it, is done as if it were a scientifically rigorous experiment on which his PhD depended.

pentaxuser