Complex sentence: RAM cannot perfectly absorb radar at any frequency, but any given composition does have greater absorbancy at some frequencies than others; there is no one RAM that is suited to absorption of all radar frequencies.
Split into simpler sentences: RAM cannot perfectly absorb radar at any frequency, but any given composition does have greater absorbancy at some frequencies than others. Different radar absorbent materials will have different frequency ranges at which they function best, so there is no one RAM that is suited to absorption of all radar frequencies.