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.
Also 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.
Does it still go wrong after that? Then replace the batteries of the scale.