An all too well known issue exists here.
To sum it up, I just dropped over 150 stamina into grinding and walked out with 4 Pyre Embers. Mind you that is after spending 40k honor on material packs. I also have 366 grave iron which I seemed to have no trouble picking up today (which is used for what?)
As if the ratio of common chests vs. uncommon chests isn't bad enough, an uncommon chest doesn't even guarantee the mats needed.
I'm sure other posts exist on this topic. Links are always appreciated.