I don't know the exact answer to that, but I have 4 Pimelodus Blochi and they seem to be a lot like them.
When I have kept them in smaller tanks or with lots of aggressive fish, they have been schooling a lot.
But when I but them in my 680 gallon tank with lots of driftwood and lots of hidingplaces, they became quite territorial and started to fight.
Later when I redecorated and took away some of the driftwood they started schooling again.
My advice would be to take all out, redecorate a bit, and then put all of them in at the same time.
Then they all start from "scratch".
(Hope you understand what I mean, I'n not so good in english...)