missing music clips on imovie ios app renders

I'm using the imovie app (ver 200) on iPad Air with more than 22gb of free space.

Exporting a 44 minutes 1080 hd movie made of pictures and 1080P movie clips, along with 7-8 different songs on the timeline. The video output in the Camera roll is great, along with the sounds of the videos clips.

But I loose the music clips after 2-3 songs (10-15 minutes in the timeline), depending of the trials I made. It seems to be memory related, but can't be sure. When I reimport the rendered clip in a new project, and add the missing songs, I manage to get a few more in the next render, but cannot complete the project and tend to think that the re-rendering will start to affect the quality of the video...

Any advices ?

Thanks in advance !

Nix