[What's_nitro?]

Even if you bought the packs new, they might be made from old cells or were just on the shelf for too long. Try cycling them with your charger at a 1C charge/discharge and see what capacity you get from them. The rule of thumb for high-power setups is 75%. So if your charger says the battery holds less than ~4000mAH, you should get new packs. If they still have decent capacity then I guess they are just overrated packs, or the problem lies elsewhere.