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.