Bests & Worsts Reviews from Amazon

I have purchased several times from this specific seller on amazon and have noticed significant variability in the products effects based on different bottles i have received. I was using a bottle i purchased a month or two ago and noticed that the magnesium L-threonates effects I was used to feeling (memory/sleep/focus) were no longer there, I attributed this to maybe building a tolerance but noticed that when i ordered a new bottle recently, the pills from it it felt how i was used to feel from Magnesium L-threonate with all of its benefits (memory/sleep/focus). I had a few pills left from the old bottle and took them again to test my theory and i felt horrible (lightheaded/dizzy) again from the old pills again! I don't know if some bottle contain normal magnesium (Not Mg L-threonate) or if they contain something else (which is very scary to think about). Take my review as you may, but something unusual is going on.