Video and Sound Syncing

So I have been using DaVinci for about one year now and no problems as such ..... am all updated too.

The latest project I am working on -

I video myself singing along to a track which is in my ear on headphones on my iphone
I import the video in with the sound of my singing.
I import in the MP3 of me singing.
I sync the two and usually all fine -

But not this time - I have about 6 videos and none will sync as there seems to be some kind of time lag between the MP3 and the video sound ..... anyone know why this might be or how to solve it? As I say I am doing nothing differently to what I have done for the past year and never had this problem.

Thanks so much

Most smartphones don't record with a constant frame rate. So, they can match, but will not always match.
You may have to correct by hand.