could b that the soil you are using is very high in newts an its overwelmed! thus causing em to die...an the strain that are goin gd are adapted to the high levels i would recommend transplanthing the dodgey ones into new soil that has vertually no newts any local garden centre should have what you need an give that a go. Also could be that the plants dont have enough breathing space in those small pots but i dont see that as being the problem as the pots are right for the size! other than that im not sure...ne1 else have any ideas?
Gd Luck!

D