Strange goings on in Garageband from Quicktime

I am recording a movie in Quicktime with the Stienburgh UR242 , I know about the left channel only scenario. My objective is to import the movie into Garageband and correct the left side only audio track into a single mono track L/R. However, when I import the movie to GarageBand and play the movie I have no sound. I have a workaround, I copy the audio and paste it into a new Audio track. Why won’t the Audio play from the original movie track ?