The hinge of Galaxy Z Flip seems to become loose or stiff.

Samsung applied the CAM detent structure on the Galaxy Z Flip and it allows the Free-stop design by digging grooves into the disk(CAM), that are connected to the two rotating axis.

Also we subjected the Galaxy Z Flip device to several rounds of extensive tests in our state-of-the-art reliability labs, including the folding test. 

If the hinge does not function properly, please visit your nearest Authorized Samsung Service Center and to check whether your device has any problem and how it can be fixed.