It sounds like the height-adjustment mechanism is still much too tight. I doubt that you reached the "highest decreased strength" mark.
Keep turning the adjustment bolt counter-clockwise until you reach the proper tension. It may require many rotations of that bolt, like a few dozen or more! (That's why the User Guide
shows the installer using a power driver to turn the bolt, though a ratchet wrench does the trick too.) You don't need to worry about over loosening or over tightening it--the bolt will stop turning when you reach the end of the range.
There really isn't a minimum weight requirement for the Neo-Flex Laptop Cart, so if it is proper adjusted, it will stay in its down-position.