Burst Arrow problem

[error656]

Hello everyone
I have problem with my burst arrow, dmg are exactly the same with every hit.

Script looks that:

local combat = createCombatObject()
setCombatParam(combat, COMBAT_PARAM_BLOCKARMOR, 1)
setCombatParam(combat, COMBAT_PARAM_BLOCKSHIELD, 1)
setCombatParam(combat, COMBAT_PARAM_TYPE, COMBAT_PHYSICALDAMAGE)
setCombatParam(combat, COMBAT_PARAM_EFFECT, CONST_ME_FIREAREA)
setCombatParam(combat, COMBAT_PARAM_DISTANCEEFFECT, CONST_ANI_BURSTARROW)
setCombatFormula(combat, COMBAT_FORMULA_LEVELMAGIC, -0.6, -50, -0.7, -20)


local area = createCombatArea( { {1, 1, 1}, {1, 3, 1}, {1, 1, 1} } )
setCombatArea(combat, area)


local xCombat = createCombatObject()
setCombatParam(xCombat, COMBAT_PARAM_TYPE, COMBAT_EARTHDAMAGE)

local condition = createConditionObject(CONDITION_FIRE)
setConditionParam(condition, CONDITION_PARAM_DELAYED, 1)
addDamageCondition(condition, 4, 2000, -3)
addDamageCondition(condition, 6, 2000, -2)
setCombatCondition(xCombat, condition)

function onUseWeapon(cid, var)
local ret = doCombat(cid, combat, var)
if(ret == LUA_ERROR) then
return LUA_ERROR
end

local target = variantToNumber(var)
if(target ~= 0) then
-- chance to burn the enemy
local chance = math.random(0, 100)
if(chance > 90) then
ret = doCombat(cid, xCombat, var)
end
end
return ret
end


what I need to change for specify dmg to (min,max) not constant value?