Cannot add audio tracks to timeline, Encore CS6
Greetings!
I am having trouble adding audio tracks to my timeline. I have several short films made my different filmmakers that I am adding to my timeline, with the intention of creating a BluRay with multiple titles. All the films are encoded exactly the same, except for frame rates. They have been transcoded from .mov files to .m4v and .ac3. Some films are 29.97fps and some are 23.97fps. All are Dolby Digital Stereo. Most of the audio pulls into the timeline fine, but a few of the audio tracks will not. When I click the desired audio track and try to pull it down into the timeline, it will not appear at all. Rather than getting that satisfying ghosty green color when I hover my other audio clips over the timeline, I get a sad black outline and am unable to drop the audio into Audio 1. I am able to get the audio track to land in Audio 2, at the beginning of the timeline. I tried to see if the audio would come through on my BluRay once burned - it did not work. So my timeline looks like this:

This is what my project window looks like, with the problem audio clip selected:

I've tried transcoding the .mov files containing the problem audio to match the frame rate of the other films in the timeline. That didn't work...plus some of the problem films are at the same frame rate as the others. I have no clue if frame rates would play a role in this issue, but it's the only thing that's different about some of the source clips. I'm baffled and not sure what else to try. Happy to provide as much info as I can, not sure what else to provide to kick this off.
macOS Sierra, Encore CS6 user.
Thanks for any help!
Jess