How to fix a boolean thats not joining?

I have a file that I’m working on that needs to be joined for 3d printing. But the last 4 pieces wont join together with the boolean. If i do it, the center ring vanishes. Is there a way to force these to become one? Here’s the file so you can see it. https://www.dropbox.com/s/99u8uf3eqd3do4l/4%20Elements%20tile.blend?dl=0

I did what you asked .blend. Just so you know, wouldn’t have it also been a good way for this thing to be printed into several separate pieces and then glued together??? I’ve seen people who 3D print stuff do this often.

How did you get it to join?

Sure, if I can’t get it to join, i could set it up to print in several pieces. Which is a good idea.

I just filled in the empty canvas of the objects with F and used a bool tool add-on to assign/merge them to the plate quickly.