There is no such thing as too much lump. With nothing else said my guess is you have the top vent almost wide open. All your heat is going straight out the top vent. Close it down to 2 maybe 3 turns open at the most. Trap some heat inside to get up to high temps. I've gotten my KK's in the 800*-900* range although I don't recommend that.
For 500* I never go more than 2 turns open on the top vent and I cook at 500* all the time.