Most likely the video recording was started too soon after the camera was powered on (One-button or QuikCapture* modes cannot be used), or too soon after the previous video recording was stopped.
In each case, the sensor pack needs a few extra seconds to initialize or save previous data before recording can begin.
Note: in this scenario, the data is out-of-sync with the video. All data is time-shifted by an unknown and variable duration, so corrections cannot be made with full accuracy.
See also the Operation tab of the sensor pack details page.
*QuikCapture for Hero4 cameras only is supported with Alti-Force software v2.11+. In this mode, the first few seconds of video will display dashes “–” in the subtitles until the sensor pack data is synced with video.