Stunted

I have got 6 bluecheese under 2x600w, i have been growing the same plants for over a year now(all off the same mother) but for some reason my last two crops have been very noticeable shorter than the ones before, i just don't get it. I have done everything the same as before, put them in at the same height( some even taller by 2 or 3 inch),but it seems that no mater what i do they just wont get taller. Apart from the stunted growth they look absolutely superb, no different than a year ago(when they were almost twice as tall. i have even tried a larger pot, but no joy. I am growing in soil and feed ghe 3 part feed. Any one have any suggestions
 
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