Hi Mark,
If one arm works OK then the spline mount should be fine as, if this moves, the park position will also alter which could cause the blades the leave the screen and wipe the air intake cowl or the screen pillar. Assuming park is OK it will probably be the slip joint thats moved and you just need to reset this to obtain the correct sweep. If you pull the blades off the screen and slide the joint one way then the other you will see that one arm will move more than the other at one extreme and more in unison at the other, so just set to get the best coverage. If your mechanism is worn you may have to extend the slot a little. Hope that makes sense.