File exists always false

I'm making a mobile app for iOS and I have files in both the applicationDirectory (inside the app) and the cacheDirectory.

No matter what I do a check on file.exists always returns false even though it is there - it's a video and it can play so it's there alright.

What is going on?

I have a string representing the path eg. "/assets/video.mp4"

var f:File = File.applicationDirectory.resolvePath(filePath);

trace(f.exists) //false