Place the scale on a flat, hard surface. Briefly press the scale with one foot so that the scale switches on and you see 0.0 on the screen.
Only stand with two feet on the scale after you see 0.0 on the screen.
Try not to move the scale and move as little as possible. Preferably give the scale a permanent place. This prevents the scale from having to recalibrate frequently.
Is it still not working? Then try to replace the batteries of the scale.