Clips not snapping with my playhead or markers...

When I'm editing in the timeline, I am trying to sync my video with the beats in my audio.. But every time I drag the end of my video clip to my playhead, it will extend past my playhead or stop a little bit before the playhead; I can never line it up exactly with my playhead. And yes I have switched my session to 'audio time units' and I have been turning on and off the 'snap' tool, but still can't seem to get it to work correctly. I have never had this problem and its is really frustrating. Thanks for any advice.