Hey Northo,
The problem can come from several causes, including light and nutrients.
For the light,
manufacturers recommend for LED lamps during the growth phase, you should place your lamp at a height between 40 and 60 cm and set it to an intensity between 25% and 50%.
For the nutrients,
avoid overfeeding them during growth. Always start with small amounts, dividing the doses prescribed on the fertilizer bottles by 3.
Personally, I set my MarsHydro TWS2000 at a height of 40 cm and just a bit of worm compost...
I hope I could help you, feel free to ask if you have any more questions.
Good luck with your grow...