Missing Audio Track in Timeline

I imported a video with an audio track. It plays correctly when I double-click it in the media pool, and it plays correctly in QuickTime Player. However, when I drag the video to the timeline, it has no audio track. The video was shot with an iPhone at 720p, which differs from the 1080p timeline, if that means anything.

I've had this happen before, and I ended up saving/importing the audio track separately. I'd like to avoid doing that again. Any suggestions?

I ended up opening the video in QT Player, and saving the audio track as an AIF file. However, when I tried to drag the AIF audio clip to the timeline, it wouldn't go. Now, it won't let me drag ANY audio clip to the timeline. That was in Edit mode, which is what I normally edit in. When I switched to Cut mode, it let me drag the audio clip to the timeline, though it still would not include the audio track if I tried dragging the video clip to the timeline.

So, I was able to make it work, though something is obviously wrong, either with this particular video or something I'm doing or not doing. I'd really like to know why I can't add a video clip to the timeline without losing its audio track. And now, why I can't drag any audio clips to the timeline. Other than adding this new clip, nothing has changed since the last time I was working on this project, a few days ago, when everything was working just fine.