iTunes not recognising M4V files after converting from MKV using Handbrake`

I am converting a number of MKV files to M4V format so that I can import them into iTunes and then use Apple TV to view.

Some appear to work after converting through Handbrake and others aren't recognised at all when dragging the M4V files to iTunes.

Quicktime Player 10.0 build 131 gives the following message "The document xxx.m4v” could not be opened. The movie is not in a format that QuickTime Player understands."

The movie in question is10.6g and cannot be played in VLC either.

Any ideas?

Thanks in advance.

RL