Stunted plants

Hi everyone, 80% of my plants are stunted. Majority of them are 6 weeks old in 40 oz. paper cups. They do not look like they are older than 2 weeks based on their size which is abnormal. The other 20% of the plants are very healthy, regular growth etc. Same 40 oz coainter. They have responded well to fertilizer and everything. My problem is this, why are the plants soooooo stunted, I mean.... they look like they are seedlings still when they are 6 weeks old. The 80% have "newer" promix in the cups but the other 20% have a different batch of promix. It's odd how the newer batch of soil all the plants are stunted as hell. When the seedlings were started everything was good up until the 2nd week mark, that's when some continued to grow, others not. Which is alarming because this has never happened to me in 8 years. Any idea, what's up? This was all artificial growing up until last week....

So with the 80% that are significantly stunted..... I'm hoping once they are transplanted into the ground they will bounce back.... If it's a soil issue that is. Only makes sense at this moment. 1/7 trays are good, the others are all buggered.

It's either over fertilization, lack of water at some point (which caused some roots to over dry and die), over watering essentially drowning the roots or root bound in the cups.................

poor ph in the new soil?

-HM420

the other 20% that are healthy are huge compared to the rest of them. they all have 5 finger leaves and much more developed. the 80% that is stunted has been under the same watering/feeding schedule since day 1. it seems very hard to believe they are root bound considering the significant lack of foliage. 44 oz. of soil is a fair bit to begin with.... for starts anyways