My rear wiper arm seems to have started to clip the bottom of glass when sweeping but it doesn't happen on every wipe.I can't see any damage but the arm has a bit of movement up and down by the pivot point(to lift arm off glass not where its attached to the motor)Also I haven't been working on the wiper boltThe arm seems to sit correctly on the glass when off whenIs this a common enough issue and what can i do to fix it?