This is caused when there is dirt or marks on the glass that covers the scanning device. Please try the following -

1. Open the front cover of the Photo Scanner as demonstrated on page 8 of the user manual

2. Apply some alcohol or Windex to the provided cleaning stick (do not use abrassive cleaning products)

3. Insert the cleaning stick either from the front or back and gently swipe from side to side to clean the glass

4. Place the front cover back onto the Photo Scanner and try another scan

Also choosing Colour Optimal mode will reduce any lines that may be visible when scanning in Standard mode.