After a few years we began to notice that these plates were working loose and it turns out that the two parts were cutting into each other. The place where we bought the bed had closed down,so I couldn't track down any replacements until just recently when I happened to be looking for small hinges and fixings for a toolbox project last year.
An article in a woodworking magazine pointed me to Isaac Lord where I found the needed replacement fittings. Fitting them should be very simple, just a matter of removing the old ones and fixing the new ones. While I'm doing that, I might just look at providing some extra support so that the weight of the rails and all that they support (which includes me of course) isn't solely borne by these thin metal fixings. A simple block might do the trick.