Here's another problem that I've been seeing from time to time, not sure what's going on:
You can see here that the higher files are marked as "downloading" but not actively being downloaded. The lower files of the next group (r01) are the ones being downloaded. Pausing and resuming just returns it back to the same status without changing what's downloading.
Also note that I only have 6 concurrent connections enabled, but there are 9 files marked as "downloading"