Based on what is well-known about how the FBI mistreated the rest of the OKBOMB evidence, missing critical frames on all tapes is precisely what we should expect.
FBI personnel were ordered by President Clinton, via memo, to deep-six/ignore all information pertaining to persons of middle-eastern background. The memos were also ordered destroyed on order of President Clinton. The only reason we know about this is because one agent resigned and kept the memo. He showed it to Jayna Davis.