try that, having firstly had to make it wireframe, else I end up dragging the surface they sit on ! and behold it says 42 block instances and one polysurface, so selecting poly and hiding it, and try again, 42 block instances .
Must remember that one ! click selection was my default brain decision, not drag select.
yes I get that also with such a basic test so its working as such in that way.
I have found it worked on another group of rivet tops, but then another group also failure, these are all formed from a single block instance sphere in each case, different sphere each time, then rotated about a centre, then boolean differenced with the surface they sit on/in to create rivet tops. I wonder if such is to blame, shouldnt be , but thats what I have.