Audio switching to mono on its own

Hi.
Wonder if anyone knows how to dal with this apparent glitch. I'm working with footage that is stereo and have been editing away just fine. Now suddenly as I drop footage into the timeline some sources appear as mono two tracks. I have to go to clip attributes and change it back but wonder why this is happening and how I might stop it.
Thanks.
John
Wonder if anyone knows how to dal with this apparent glitch. I'm working with footage that is stereo and have been editing away just fine. Now suddenly as I drop footage into the timeline some sources appear as mono two tracks. I have to go to clip attributes and change it back but wonder why this is happening and how I might stop it.
Thanks.
John