Its not particularly unusual for a mechanical tape DFDR to skip a whole cycle. This can be due to the tape sticking to the spool and moving away from the recording head, especially if the recorder is close to its 8,000 service. Also, immediately prior to an accident when an aircraft is experiencing high 'G' loads, the tape can be moved off the recording head surface by the 'G' forces and fail to record anything. In normal service mechanical recorders suffer an error rate of up to 1 0r 2% of the sample, anything higher than this being a cause for removal. FAA require a sample readout to be made every eighteen months and the UKCAA once a year.
Solid state recorders or SSFDRs are much more reliable and rarely have a sampling error much above 0.01%. I don't know which type was fitted in the accident aircraft but would suspect it had the older mechanical model of DFDR.
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Through difficulties to the cinema