Hey could be many reasons just a bad pheno for instance. a slow growing plant. Nothing to be fearful of most of the ones that are slower to start turn out to be the best. Give her a little bit of your growing skill and plent of love and she will do you proud :) Could also be her reaction to the nutrients in the soil most plants even if they are the same strain will show different traits this also means they all react differently to feed or nutrients. :) Overall they are looking healthy I'm sure she will catch up :)