It could be that you want to make the multicam to fast. When files with audio are imported, thumbnails and wav forms have to be rendered first (see the progress monitor). When you do not wait before these are finished and try to make the multicam clip, FCPX waits with finishing the multicam for the rendering to be finished it self and than makes the multicam.
This is how i do it, i import the clips and go out for lunch or coffee. When i come back and the thumbnails and wave forms are finished rendering, making the multicam clip is done within 30 to 50 seconds. Even with 8 camera's full HD.
The wave forms are always the brake on the whole fcpx project so really waiting for these to complete, it might give you benefit while continuing your project.