The rear calipers rotate one of the pads on
a screw to engage the parking brake.
If one just put new pads on I would suggest
If not new pads, I would suggest looking
at the inner pads to see if the are just plain worn out.
Inspect the mechanical linkage on the rear calipers
while someone engages and disengages the parking brake.
If the levers move correctly then I would suspect the rear
calipers and/or pads.