Hello -
I commented on this about a year ago and I've never found a solution to downloading in Field Maps. I have not been able to find the correlation for why it works sometimes but not others. I spoke with Esri tech support about it a few weeks ago. He was able to rule out for me (for my specific situation) that internet was not the issue, photo syncing was not the issue. He suspected (in my situation) that my database was corrupted. The tough part is that we really don't have the time to be running through all the troubleshooting options while the crews can also work from the online map. Other suggestions provided: check for field maps update in app store, sign out/sign back in, verify internet connection for download/sync (the typical troubleshooting). My crewman ended up working on the live map. (I basically told tech never mind, since I don't have access to the tablet, I don't have direct contact with the sub contractor collecting, and the project has since completed)
He forwarded me this documentation. Based on my conversation with the Esri rep, creating the work areas ahead of time in Field Maps is the best practice for creating offline maps ahead of time. I have yet to try this.
Take your web maps offline with Field Maps
Not sure this post will help anyone but maybe it will!