If brakes have been used excessively when first installed it can harden the surface of the brake pad or this might happen to some cheep pads which have a low amount of copper , you can solve this problem with a brake service <
remove pads and clean the vent cut , sand using 60 , 80 g sand paper (pads, slider, disc) , apply copper grease (anti seize) carrier and slider , ensure all retainers ,clips are sitting nicely .
🙂