Missing File

:huh: Hi - I'm new to FCPX but old at FCP7. I've imported a couple of cards recorded on a Sony A7S and now I'm getting a Missing File warning on one - even though I have already used it yesterday in my project. And another clip just refuses to be imported into the Event . So I started a new Library and imported both into there - fine. But if I move them across to the Event where I need them, then I get Missing File. Any explanations gladly received. Cheers