A2plus is designed to import parts, each one been designed in a separate file. If you import your single parts into a assemby file, you can apply constraints and this will work.
Basically the files need to be in separate files so if you have 5 bolts in an assembly the bom can report 5 bolts are used. if you upload a file perhaps we can take a look.
Here are the parts back. To create the separate pieces I highlighted the three parts that I did not want and then did a "save as" with the file. I found out that there is not a way to line up the round parts in a slot so I made a rectangle with a hole in it and centered the bra to that. I made the rectangle the same width as the slot. None of the constraints are movable. I'm hoping between this example and the how to do videos you can make it work. When you put the first part in it is "fixed" in space. as long as you add a constraint to a part that has one constraint linked to the first part you should not get the cannot move error.