I had a similar situation w/ old Mg mix and a similar issue getting my Mg levels up in the tank.
Part of my issue was my IO salt mix was diluting my Mg levels every time I did a water change. (I had not been paying close attention to the freshly mixed water's Mg levels.)
That being said, I could swear that BRS changed their recipe from back in the day to now. I manually mixed up a fresh batch last week from separate containers of Mg Sulfate and Mg Chloride, following their instructions, and it felt like I was using 4 times the mix compared to what I used to get in the pre-packaged packets. I have also noticed a difference in the "thickness" of the mix when pouring it into the tank.
In any case, my Mg levels have coincidentally leveled out.