Arturo Mediano teaches courses on EMC/EMI and Signal Integrity for Besser Associates. He is a frequent contributor to industry magazines and journals on these topics. This article appears in the "Practical Tips" section of InCompliance magazine.
Ferrites for EMI suppression are usually chosen looking for high (resistive) impedance at the frequency of interest; but, sometimes, that ferrite is not working as expected. Perhaps you have saturation effects?