I found that if you have changed anything in the a specifc layer or the Webmap, and you are trying to redownload the offline map to reflect these changes you need to repackage and update the layers.
On ArcGIS online, go to the webmap, in setting choose Manage offline areas, and on eahc map you have created, you will need to refresh the layer you edited. I typically will refresh all of them.
Then on Field Maps, log out, log back in and try to re-download.
This worked for me. Good luck!