If I understand what you are trying to do…
It appears you have extra planes mixed in the top tab object, as shown in the lower part of the screenshot, which is confusing the Boolean subtract. I moved them out of the way.
To repair the subtraction attempt, I exploded the upper female part, remade the 2 planes with holes in them that could not be simply deleted because they were cut on the edge. The hole subtract tool can’t delete a hole that lies along an edge, AFAIK.
Anyway, after adding the two planes to repair the failed tab attempt, and made it a solid again…
Then, I was able to subtract the tab part.
As a thought experiment, you can select the parts you want to keep, and hide them. And then see what remains. Of course, you need to be careful with groups.
Rhino also has a _SelSmall for selecting small stuff. Microplanes, we used to call them, back in the day.
I hope that helps.
BTW, as long as you know, most of your figure is not a solid, just the tab areas. If you start attaching planes to solids, things might not work out well. You can do Boolean subtractions mixing solids and planes, though, which is handy for cutting things. I usually use booean 2 objects for this.
To attach the tabs to the figure, you might need to firstly add surfaces on each half to enclose each half, and make them solid–before attaching the tabs.
Misc things to ponder:
If you are repairing and remaking a solid that should be created but fails to be created, check the dir of the planes.
Be careful of things that touch or graze just barely on a shallow angle, such as a circle along an edge, which is tangent to it.
I imagine that you will have a lot of coplaner and nearly coplaner solids when you are done, on odd angles., and Booleans can be challenging that go in and out of one another more than once.
If you are doing Boolean adds on strange parts on odd angles, it’s often better to make one thing is quiet well enough into the other instead of trying to line them up. Regardless of what CAD system, there is floating point limitations, If both parts are turned/translated to 120 degress, the X- coordinate of an edge might be .000000001 and the other might be at .000000002.
…In other words, you can either spend a lot of time lining the bottom of a telephone pole up with the ground to add it, or just shove it into the ground and add it.