Every time there is an update of e/os, I need to restart the download process multiple times. After a few minutes it stops again. I have to do this at least 4 times to fully download a new version. This is very annoying. Does the download thread crashes? Does it not retry after a dropped package/bad wifi? Or is there an issue with the download servers?
It seems to me that it simply does not run in the background. If the screen closes / locks the activity stops.
The best method is to pull down Quick settings and activate Caffeine tile, preferably set to ∞. (Infinity will cease at reboot)