Not sure what motor mounts you're talking about, but on a couple of them (not the ISOs) that attach to the cradle, I found that I couldn't get them out, or particularly, back IN, unless I did so at a certain point in the process of putting the bike on or off the centerstand - the holes don't line up correctly except at that point. So I had a helper slllloooooowwwwwwlly put the bike on the stand while I inserted the bolts.
This may be way off base but I throw it out there just in case.
Best of luck.