Are you referring to the end of the pitman that connects with the knife head on the sickle knife?
Take a look at the diagram below of the regular cutter bar.
Check the "ball" on the knife head (Key 15) for excessive wear and replace the knife head if necessary.
Take a look at the diagram below of the sickle pitman.
The rear strap (Key 1) and front strap (Key 15) are held in place by tension on the latch fork (Key 17) by the spring (Key 16).
Check the spring tension and replace the spring if necessary.
Hope this helps.