If one of the rear wheel does not spin freely, It has somekind of noice and a drag. It is only spinning slowly 2-3 revolutions and then stop. It doesn't either help switching the wheels and that make no difference for how long the the wheel spins. Then you might need to center the brake collar. It is easy and takes you a minute or two.
Just follow these simple steps:
Step 1: Locate the adjustment hole in the brake cover to access the M5 screw with a 4mm allen key
Step 2: Turn anti-clockwise to loosen the screw 2 turns
Release the "brake collar" M5 screw 2 turns
Step 3: Apply and hold the brakes locked
Step 4: Tighten the M5 screw with a 4mm allen key
Lock the brake lever fully and hold it. While holding the brake lever, tighten the "brake collar" screw.
Step 5: Test the brakes by spinning the wheel and checking there is no rubbing between the brake collar and brake surface.
NOTE: Brake lever in picture may be different to your model.