What is wrong with my rummy nose tetra?

I have 7 rummies in a community tank and since i got one about a month ago nw... he is really tiny... wont school with the others and used to be very aggressive and turn on them but now he just sits in the plants i thought it was because they were near fry but the others are fine now and he isn't... i tried feeding them frozen blood worms for nutrition but still the same what do you think is wrong?