Why iTunes u automatically exits to home screen when I download a material of a course? I have downloaded all except for that material.

I have subscribe dor a course through iTunes u and downloaded a lot of materials from that course. One of the material (a video) can't be downloaded when I click download irrespective of how many times I try. It's a 45 minute video and a very important content from the course. Of note, I also downloaded other 50 minute videos and they are fine. I really don't know the reasons dor this. Please help!