External Audio Does Not Line Up With Video


In Runtime
Hi all,
I've recently started a YouTube channel for my school (Café Veritas, go check it out!), and I'm having some issues with the audio for our first video. We used an external condenser "snowball" microphone for our audio, and recorded it into my Mac Pro using Garageband. For video we used three different iDevices in order to have three different camera angles. Now, when I line up the audio with the video in iMovie (we used a handclap so I would be able to spot where I should line the scratch track on the video and the microphone audio up), it lines up and sounds perfectly... for about a minute. Then, the audio proceeds to deviate from the lineup, and if you have both tracks (the scratch track from the video and the microphone audio) unmuted at the same time, it's more obvious, as you begin to hear an echo. By the end of the seventeen minutes of the video, the audio doesn't line up at all. I've tried converting audio formats on both the scratch track and the microphone audio, to no avail. So my questions here are:
1) Why does this happen? (I'm not an audio expert here and am genuinely curious about this)
2) Can I fix it? If so, how?
3) How can I prevent it from happening again? (Use a different microphone, etc.?)
Thanks as always.