If you are cutting aluminum, be sure to use a high helix end mill (I suspect you probably already are), since they will help lift the chip out of the slot.
You say you have tons of flow, but you may need more pressure to blast the chips out. You may also experiment with different nozzle configurations. The VMC I used to run, had a hand held garden hose with a spray nozzle that could be used to augment the flood coolant to periodically blow the pile of chips away.
The flow and pressure from the hand held, was equal to what it would be if it actually were a garden hose (to give you a point of reference). Of course this required an extensive enclosure to contain the spray.
Hope this helps.