I'm puzzled how this could be... Perhaps a bead broke off, I suspect when I stepped on the rosary once.
I'm now considering the choices of just adding an Ave Maria or buying a new memory rosary (they work pretty well for me). Adding an imaginary bead is no big deal, but I'll have to try to see how it goes.
Getting a new memory rosary is cheap, but letting go of the current one poses some pondering. First, I've never disposed of a sacramental before and I can't decide between burying it or burning it. Secondly, I really like my rosary, worn by my fingers going over the beads...