Why should the THC be lower ? well there is no reason for that , a stabilised strain is suposed to have an average thc range depending on the each individuals.
So for the one that is not flowering then it must be because it's not an autoflowering , if you flip her to a 12/12 schedule she might flower. Sometimes autos are not autos it is because autos are crossed with photoperiodic strains to increase their thc level and sometimes the autoflowering genes is not transmitted to your 3rd plant