DFDR recording is VERY complicated.
For instance, some parameters will be refreshed 5 times per second in the DFDR, and be recorded only 4 times per second, so you skip one in every 5 possible recording.
Parameters are not at all refreshed as much as you would expect them. The best ones being refreshed 8 times per second (they're very rare)
For instance if you push your button "event marker" in the cockpit, in order to mark an event in the DFDR, it will only work if the parameter is refreshed (written) while you're pressing it. Since it is (say) at 1 point per second, you could be unlucky, press it for 0.9s and miss it !
To finish with, some parameters will be refreshed very regularly but the parameter itself can be late due to some reason (gear squat parameter can be as long as 2s late)