iMovie fails to properly export... memory?

So I made a bunch of movies and started exporting them (sorry, the term now is "share").... The basic editing I had done exported properly but none of the noise reduction, cropping or titles exported to the final product. What the heck is going on?

My theory - it takes so long to export a movie that I started working on a new movie while the current one was still exporting by deleting everything and starting the new one, then repeated this 4 times and went to bed, thinking these stupid things would be exported by the morning. The result - a massive waste of time and now I have to start them all over again just to include a simple title.

The question - is this expected behaviour? Is this caused by iMovie not having enough memory to handle more than one export? Is the fact that I deleted everything the problem?

Very annoyed.