When printheads are defective they leave a distinct thin line through the card. There is no fix for this issue and a replacement is required.

If a printhead is producing a larger (wider) line or a different defect, it is likely a different issue that can be fixed through other means. 

When you are handle the printhead, be very careful to not touch the black bar. This is the part that heats up and contacts the ribbon and PVC card. Rings, nails, etc. can scratch the printhead and render it useless. 

If white lines are printing on the card, examine the printhead for scratches or chips. You can use a clean cloth to wipe off smudges or debris, being careful not to damage the printhead in the process.

After cleaning off the printhead, try printing another card and see if the problem has been fixed.