Marking mail as "Not Junk" doesn't work

Dear All,

Much of my email is suddenly going into the junk mail when it shouldn't be. I have repeatedly marked the mail in question as "not junk", but the next day it goes into the junk again and I have to mark it again -- which doesn't help. I'm working with macOS Sierra 10.12 on an iMac Retina 5K. I know that I could add these emails to my contacts, but I don't want them there -- I just don't want them going into the junk every day when I have indicated that they aren't junk. Anybody else out there having the same problem which is something probably happening with the latest Sierra update? Thanks.