I had a similar problem, but it came out of it. It stayed a little smaller than the rest but was a producer all the same. Make sure that all the basics (water, soil, light, ph, PPM, etc) are all within code and the plant will come out of it.
AK47/Blueberry/Auto Flower
First time on this strain but I have done a few grows with greater and greater success. This round has me puzzled. A previous planting had a similar issue when one plant (California Hash) was moisture restricted. It was yellow and a bit stringy compared to the other...