I've had this happen to me on several occasions. The AI reinforcements spreads out just about from end to end a part of the map.

-SOME of the units advance toward my army.
-Or the AI tries to do a REALLY long flanking move with a 1 or 2 units on each side while the rest stay back, (some even stay right behind the red line).

What I do though. Is I move my consentrated force in a horse shoe formation towards the largest consentration of force the AI has on the map. Then as soon as I start scermishing with that force a large portion of the other wandering units strat comming towards me from 3 sides, while others hold back only to advance later in the fighting.

I play VH/VH with Makedon so that may have something to do with the AI finaly engaging. What difficulty do you play?