These are purposefully tight to support the weight of a monitor, so the tension bolts can be stiff the first time you adjust them.
To loosen the bolt:
- Install a monitor onto the arm before attempting to adjust, then use the monitor itself to try to move the joint up and down the first few times.
- If that doesn’t work, lower the monitor down onto the desktop, then place something small and stiff -- a block of wood, a brick, or a hardcover book (whose appearance you don't care about) up against the hex key while turning it counterclockwise. This gives a larger surface to push against and allows you to put more force into turning the hex key than you would normally.