HP ProDesk 400 G4 Small Form Factor (SFF) - Maintenance and Service Guide

Mouse will only move vertically, horizontally, or movement is jerky.
Cause Solution
Mouse roller ball or the rotating encoder shafts that make contact
with the ball are dirty.
Remove roller ball cover from the bottom of the mouse and clean
the internal components with a mouse cleaning kit available from
most computer stores.
Solving keyboard and mouse problems 75