Sharing to itunes does not work

Using latest version of iMovie. Imported 90 minutes worth of video and assembled 4 movies. One has sucessfully been shared with iTunes the other 3 cancel during the process with an error message. The share times is very long sometimes overnight required for the longer clips. I am using a Mac Mini (mid-2010) with 4 Gb of memory. My iTunes libary is on a network drive. Any thoughts on why one movie will 'share' and the other three will not? Thank you in advance.