Permissions for Shares - access denied

Brand new EX2 Ultra, 2 mirrored 4TB drives. I have 2 users (plus admin). I created 3 non-public shares, one for each of the users and one to be shared between them, and assigned read/write permissions as appropriate. I was able to map my personal share as a network drive, and copy files/folders to it. When it asked for my network password, it accepted it and mapped the drive as expected. As the same user, who has full read/write permission to the shared share (there’s no easy way to say that, is there), when I try to map to a network drive, it asks for network credentials, and despite my confirming my password multiple times, both on my computer and within the WD dashboard, it tells me access denied. I don’t want to make the shared folder public - I want to restrict it to these two users. What am I missing in mapping that share? I’m running Windows 7, so all the “here’s how you do it in Windows 10” resources I found aren’t helping. Thanks for any suggestions. I wasted two days on this over the weekend.