When you have a multiclip already in a sequence, be aware that if one of the clips (or more) was renamed since it was edited into the Timeline, FCP can get confused and attribute TWO file names to one clip.
This will result in all manners of strangeness. For one, even though all the footage in your bin will be properly connected, a multiclip in the sequence in your timeline may still report a red bar over it, and if you hover your mouse over the red bar you'll see FCP complaining that one of the clips is offline...but it's not...it likely it just has two file names attributed to it. One of them being the old file name that's no longer available to FCP.
The solution is to 2nd click on the clip in the timeline, select Reconnect Media and then after connecting the properly named files, ignore the improperly named one(s). You should see the red render bar turn green nearly instantly.
This can happen if the files were initially named incorrectly and an editor renames them from within FCP or in the Finder late into a project's life.