There is a bug in Suikoden I: Characters in each row (front and back) recieve (at most) the experience received by the left-most character in that row. This will cause lower level characters to receive very little experience if placed to the right of higher levelled characters.

To work around this bug place your lowest level characters to the left of higher level characters in each row. Each row is handled independently so you can order them Low to High in the front row and then Low to High in the back row without either row interfering with the other.