iMovie 10.1.14 - audio falling out of synch from video
When I synch a video clip to an audio clip in iMovie, the audio gradually falls out of synch. I have a two minute audio clip that for the first 30 seconds is in perfect synch. However gradually throughout the remaining video the audio falls out of synch to the point where it is completely obvious by the end of the clip.
Audio and video were recorded simultaneously. Any ideas what causes this to happen?
iMac 21.5″, macOS 10.15