Might just need the wiper tightened up on the fitting. Other wise might be the link arm disengaged, Very easy to fix. remove wiper from fixing,Turn on wipers. look at centre , fixing of faulty one, does it turn, If so just needs arm reconnected and tightened, IF not, linkage disengaged, usually a lenght of metal connecting the two. Check linkage, probably needs a new clip fitted to hold together. Job done.
SD
December 2012