The new spool holder design uses skate bearings to reduce friction.   The end caps, however, are tight and sometimes break.

If so, the STLs to print a new rod and cap are listed at the bottom of the Spool Holder product page.  

Click on Products at the top of the website.

Click on 3D Printer Accessories

Click on 3D Printer Acrylic Spool Holder

Scroll to the bottom

To insert the end caps place a bearing on an end cap and place it into the corresponding rod hole. 

Next place the rod vertically on a table and press down on the other end with the palm of your end.  Use steady pressure, but it may take more force than you think.

The temptation is to try and leave a little space so that the bearing spins freely.  The problem with that is with zero friction the movement of the head can start the filament spool spinning so much the filament comes unraveled.  The design is to leave a little friction to prevent that from happening.