re-attach VESA display adapter

My 27" LED Cinema display has been attached by the VESA adapter model A-1313 to a flexible arm holder bolted to the side of my desk. I removed the monitor from the flexible arm by loosening the four long screws, one at each corner of the square adapter cover, and set the monitor face down on my desk. My purpose was to reattach the monitor to its original curved mount that would stand on my desk. But I have been unable to do this. The adapter kit consists of 1) a mounting bar that I had earlier attached firmly to the back of the monitor case by 8 small torx bolts; and 2) a square cover plate that I had earlier attached to the mounting bar by two bolts, one each on the left and right sides, while a third smaller bolt at the top center of the adapter cover plate also attaches it to the mounting bar. Intending to remove the whole kit, I first removed the two bolts at the left and right sides of the adapter's cover plate. Then I removed the bolt at the top center of the cover plate, which allowed the cover plate to be lifted off the mounting bar. Failing to remove any of the 8 very small torx bolts that hold the mounting bar to the monitor case, I decided to abandon the plan to use the desktop mount, and resolved to reattach the adapter's cover plate. But after much trial I have been unable to do this.

With the adapter's cover plate removed, I can manually fit the two left and right bolts tightly back into the screw sockets of the mounting bar. But when I replace the adapter's cover plate and insert the two bolts through holes in the cover plate, they cannot be screwed securely into the sockets from which they were removed. Without the bolts secured, the cover plate is held in place only by one small bolt at the top center of the square cover plate, which is not firm enough to bear the entire weight of the monitor if the adapter is reattached to the arm. What can be done to get the VESA adapter properly re-installed?