Can't seem to filter junk mail

I am getting really frustrated with how Mail handles junk and spam. I have marked the same messages multiple times as "junk" and still it gets sent to my inbox. On the other hand, lots of regular senders that I have been receiving email from for years will suddenly get sent to the junk pile. Why? I never asked for that. At times whether something gets labeled junk just seems arbitrary and almost random.

I'm aware that many spammers constantly tweak their addresses to get around filters. That's not the kind of thing I'm talking about -- this is the same content from the exact same sender. Sometimes it gets filtered. Sometimes it doesn't. And the problem of perfectly legitimate email suddenly getting junked is maddening.

I guess my questions are: 1) does anyone know a way to improve how Mail handles this; 2) am I doing something wrong, or not doing something I should; and 3) is Outlook for Mac any better at handling this kind of problem?

My Mail program is 11.2, my OS is 10.13.2

thanks in advance.

Mike