Plant growing slowly?

Are they clones of the same plant or are they from seed?if their from seed you may have two different phenotypes,meaning one plant can be tall and and wide and the other phenotype could be short and compact,if your doing the same thing to both of them and there in same conditions I'd bet u have two different phenos,I could be wrong but that's my guess

The plant looks healthy and I wouldn't be worried about it's size - 90% of the plant size is linked to genetics. You said the plant next to it is a different strain, so it's not surprising they are different looking. I currently have four plants of the same strain in the same hydro reservoir, yet two are large and two are XL. Like brothers and sisters, they won't all look exactly the same. One thing I see in your photos is that the leaves are looking a bit crinkly and some have canoe'd (the sides are turning up). This has probably been caused by over-watering. I'd recommend ensuring that the pots are drying out sufficiently between waterings. Simplest way to check this is to lift the pot up every day - you will very quickly learn what the wet weight is and when the pot is feeling dry, as it get's super-light once the water has been used up. Good luck!

siete in molti ad avere paura della pianta troppo corta invece io quando ho la pianta corta sono felice perche non avro problemi con le altezze comunque devi ancora far partire la fioritura ed è li che raddoppiera' o triplichera le dimensioni la fotoperiodica che sto facendo ha raddoppiato l'altezza in 2 settimane dal passaggio a 12/12