Not all seeds are destined to make it.
She will not affect the others, but probably no point going on with it, especially if it is already 2 weeks old, things are not going to turn around.
Try using a specialty premium "seed raising mix" next time, the substrate you are using seems to contain lots of wood chips, which can be very acidic and are very low in nutrition (but a cheap additive for the manufacturer).
The results from using the premium seed raising mix will astound you!
Just remember -
Premium ingredients = premium results!