Does anybody know why one of them is not growing a...

Patience growmie. Every plant is different than another. It'll probably catch up with the rest but at a different pace ;)

You could also move your light higher to help her stretch but keep in mind so will your others! I have a shorty in my grow also! She went into flower at 3 weeks and is now the show stopper! The whole stalk of this plant is bud! But yeah you can remove it or let her go. I would let her go and see how she does. She could catch up and pass the others while in veg. I'm running two grows at the same time and my second grow is 3 weeks younger than my other tent and they have caught and passed my other grow due to mistakes and stress. So yeah I would give her a chance. Best of luck and Happy Growing!😎🌱💚💪💪

Organoman is spot on. Not all seeds are equal. Some just never end up being a biggie. But don't despair, it'll still produce something. Just don't waste your nutes on it. I stuck mine in plain potting mix so i could feed straight water. I'm going to get a little sesh out of my runt. You can see it in my diary. If you got the room keep it going. Up to you, but you've done everything right sometimes it just doesn't turn out

Not all seeds are creatd equal, this is why it is important to plant spares, this way when you get a slow grower or a mutant, you can have healthy back up plants to choose from. I always plant more seeds than I need and only grow on the healthiest examples and cull any strange seedlings.