Hi Sophie,
How are you doing? This is actually quite normal and often the line will get darker as the pregnancy progresses. I have also seen cases where the line stay quite faint throughout and there has still been a very healthy pregnancy.
Sometimes if you are drinking a lot throughout the day, it can dilute the urine and then the pregnancy hormones that are secreted in the urine don’t show up as much so this could also be a reason for a fainter line.
Hope that helps?
Fran x