Maybe you should post your macro to see if there is maybe a fault concerning the order.
Just to clear it up:
Hotstreak (HS): is an InstantPyro
PoMPyro: is an InstantPyro
The idea behind this macro is to save both with AT and double them.
So the most important thing is: PoM has to be activated BEFORE AT!
Another fact is that a HS Pyro doesn't use the PoMbuff.
Sometimes (due to Serverlag i think) by resetting AT you start a HardcastPyro but have the 2 InstantPyros resetted and ready.
To avoid this you can just do sidesteps or jumping while chaining Pyros.
I have/use 2 Hotkeys for that:
1 - Macro (PoM->AT->Pyro)
2 - Pyro
So i do: 1 2 (2) 1 2 (2) - while donig that i spam Q,E for sidesteps and avoiding the hardcast.
ANOTHER IMPORTANT THING NOT MENTIONED:
Start this sequence not with HS only, wait for HS/HU
Be aware of Crits an dont do all Pyros if there is a Crit
After a Crit do just 1-2more and the combust
Hope that do clear things up for you, i am not native speaking english, hopefully you could get what i mean
Have fun and keep on trying