Surely this can't be right?
The problem may have arisen because when you last worked for a local authority the main pay spine had nine points. During the intervening period this was reduced to six points. If you left with two years of service this would translate from the old M2 to M1 on the current pay spine. The local authority has discretion to award extra points for work "other than as a classroom teacher", but not apparently for such work. It seems that you might indeed be better off going back to supply work unless you can find a way for the exercise of some discretion about your work over the past three years.