Cubase mastering cuts off first bit of audio!!

Hi folks don’t know if anyone can help me on this one…

When I import a cubase 6 mixdown into the Cubase 6 mastering facility the imported mixdown is minus the first few seconds of the file. The whole file is there, I know, because I have opened it in Windows Media Player and played it. This only happens with one particular mixdown! Others work fine. If i open it in Wavelab 7 it’s also fine. Want to open it in Cubase mastering first as I like something it does. Hoping someone might have a bit of insight, thanks.