Complex sentence: This proved to be unreliable, needing repeated fixes for which Harrison was not paid, so around 1722 he modified the escapement by putting a hinge in the middle of each arm of the anchor.
Split into simpler sentences: This proved to be unreliable, needing repeated attention for which Harrison was not paid. So he modified the escapement (around 1722) by putting a hinge in the middle of each arm of the anchor.