Most likely you got unwashed coir. Most coco coil sellers "claim" their coir has been washed. in my experience with coco coir I, like you, found out the hard way with expensive seeds. It needs to be washed really, really good. Coco coir is really salty. You can buy washing solutions. Advanced nutrients, Botanicare, and Fox Farms sell these and work really well. Also use nutrients made exclusively for coco coir. Hope this helps. Remember, Wherever you are, that's where you're at.
Flush your medium with PHd water. Check the run off. A lot of things could cause stunted growth. And wait for about a day or two, to dry em out. Maybe your root system is underdeveloped. to speed up the dry wet cycle. Good luck