try changing one input variable, i dont have either machine or grinder.
but others are not gong to be using your beans anyway so the grind setting could be different just due to the beans
have you just tried grinding finer? some grinders the numbers are just a gauge and zero is not zero. How many numbers between "8" and locking the burrs? (do they lock at zero?)
as for the 1st tamp, can you just tap tap it on the bench (tamp mat / cutting board etc) instead of tamping. you might be creating some channeling etc?
how many kilos of beans have you "bean" through?
practice practice practice changing a little at a time might help you.
have you just tried making shot after shot and sinking them?
i know it might seem a waste but...........
where is the 21 sec measured from? 1st drip? or 1st start the pour?
this will help as well