Aeroponic seedlings slow growth

I had a fairly big grow set up a few years ago with mothers and everytime I started a new plant, I cloned from one of my mothers.

I have recently started another small grow with limited space. I plan on growing two autoflowers in an amazon aeroponic unit, and once they are 5 weeks old, I plan on cracking open another two autflowers an aeroponic cloner, which will then be transplanted into the larger amazon once they are roughly 4/5 weeks old and the original plants have been harvested.

The problem is, I am having very slow growth from my first two plants. I cracked them open in paper towel and waited until the tap root was about 40/50mm long, then put them into the aeroponic cloner straight away in a neoprene collar, and didnt use any nuits, only water with a ph of 6.

For the first week they had decent growth, the first leafs emerged, followed by the second set, then the growth slowed down. I recently added some 1/2 strength nuitrients and I have noticed a slight increase in growth but not much

Should I have added nuits earlier, say after one week or so? I didnt do this as I felt the root system didnt look developed enough and I never added nuits until I had a very strong root system when I cloned. The plants are now 4 weeks old and should be much bigger and ready to start flowering, I hope Ive not wasted them as the autoflower seeds are pricey

The temp range is : 20 - 28 degrees C
Humidity is : 30 - 50%
The roots are sprayed once an hour
Ph is : 6

 

They certainly are looking healthy, and if they were standard cannabis I wouldnt be worried. My only concern is that they are autoflowers and the plant should be going into flower very soon. 

I would have expected the plants to look something like this after two weeks