How long have you had the plants, it could be that one is just a later bloomer than the other. You could also be having a nitrogen deficiency problem or there may be an allelopathic interaction between a neighbouring plant and the stunted plant. Though, I'm doubtful of this because their is only one plant in between the two plants.
Though the one plant could be enough to buffer per se.
What do others think?
Feed the soil, not the plants.