I'll take a WAG here and say that your basic problem lies in the fit of one bracket over the holes of the other. The first has a radius of X the second Y. When you move Y out and on a bracket the holes of X seem to be over them. Sorta half and half not allowing you enough room to place the bolts through any of the holes. If you were going from say 9" to 15" rotors the size change would be big enough to accomodate this but from perhaps 10-11" the offset change is just too small.
