Cheap screws can be notchy. I wonder if customers complaining about backlash and demanding refunds cause the manufacturers to use tricks to tighten them up beyond reason.
I bought some cheap 20mm profile rail that is notchy. It glides tightly when you go quick but sticks when you go slow. It's wringing with oil, could be grit. The notch will support a 2.5kg weight but not 5kg. Luckily the finished block of 4 will be supporting more than 20kg so I should be okay.
Yesterday I bought a 500mm screw with nut and end machining for £16. Amazing bargains to be had if you are prepared to fix a few minor issues