Problems syncing multicam shots

Hi, I am trying to sync up two shots but fcp does not allow it and I can't understand why.
My process: I select the 2 shots I want to sync then R click> select new Multicam clip - but it just beeps and will not go any further. I have also tried syncing just "synchro clips" but this is greyed out and does not work.
Please help - thanks K

What codec/format are the shots? What camera were they shot on? Duration of each?

The 2 shots are ±4 minutes long.
Codec H.264, Linear PCM
All material was imported by client and I'm working off his computer. Apple 10.9.5

What version of FCPX?

Are the two shots in the same event?

Yes the two shots are in same event. These shots have been previously multicam synced but I want to do it again because the sync is missing the last sound track at the end.
The picture contains 3 classical music pieces (3-5 minutes Long each), the last piece had not been previously synced up and so I want to add this track of music to the picture.
I have tried syncing other pieces from the rushes collection but nothing is working - I am wondering what I must be over looking ..

Can you post some screen shots so we can see what's going on in your browser?

If you've already made a multicam clip you can add an angle to it and drag in the audio clip.

I will try this, and let you know how I get on - thanks. K