External Audio Runs Out of Sync after a few minutes

https://www.youtube.com/watch?v=v_LGXjKiRM8

I use a dslr to record video and audacity with a separate mic to record audio (naturally i mute the dslr audio and use the microphone audio). The audio that is connected originally to the video clip plays back fine, but the audacity audio that I record runs out of sync with the audio from the dslr after a few minutes when they are perfectly synced up from the get go. I didn't have this problem with windows, and if i were to put both the dslr video and the microphone audio in a windows editor they would sync together fine.