PQR skip actions

[fulga08]

I am trying to save pyro / ib for right after the fireball cast but it doesn't work fine. Most of time is working but once every few casts it just skip even if i have both aura up, it does not cast pyro right after fireball. During tw/bl it only casts fireball. I try setting different custom latency and different ability check delay.
First i had this split in different actions with the spells id set and returning true for cast. Had same problem so i simplify and made a single action.

local Pyroblast = UnitBuffID( "player" , 48108 )
local HeatingUp = UnitBuffID( "player" , 48107 )

if last_hardcast == nil then last_hardcast = 0 end
local cast_name = UnitCastingInfo("player")

if cast_name ~= nil then
if cast_name == "Fireball" then last_hardcast = GetTime() end
end

if GetTime() - last_hardcast < 0.5 then
if Pyroblast and HeatingUp then CastSpellByName("Pyroblast") return true end
if HeatingUp and select(2,GetSpellCooldown(108853)) == 0 then CastSpellByName("Inferno Blast",nil) return true end
end

CastSpellByName("Fireball",nil)
return true