The linear bearings have become loose in the gantry carriages.
Constant forces within the v1.0 & v1.5 gantry carriages sometimes causes the plastic retaining the linear bearings to weaken, causing them to become loose.
This was resolved in v1.5a of the gantry carriage design which now use a larger surface area to retain the bearing along with a retaining clip.
Kits shipped after 15 September 2015 contain the revised gantry carriage design.
For earlier kits, the new carriages are available for purchase through our online store here: Link
They are also available as part of the Upgrade Kit v1.0 -> v1.5: Link