Delayed recording

I'm trying to record some vocal tracks, so I start with one take and then record the vocalist singing over the ffirst take.  The problem is that, for some reason, some of the tracks recorded--all starting at the same time--aren't appearing to have started recording at the same time... some of them seem to be delayed by about a second when I know for a fact that the recording of the track was not recorded a second later than the other takes.  Any idea why this is happeneing and how I could fix it?  I'm using a Yeti Blue USB microphone and CS6