3x 250mg magnesium oxide at once sounds like a lot at once. Even one 350mg magnesium oxide pill completely fucks up my stomach/digestive system, I get stomach cramps diarrhea you name it. Perhaps you had problems with absorption because of all the magnesium you took beforehand and experienced a diminished roll? I don't think taking magnesium would have any effect on your brain that would diminish the roll atleast, so my bet goes to slower or somehow impartial absorption.
With magnesium oxide you should be spacing your dosage and taking it with food anyway (I am making the assumption you didn't eat much because you took mdma an hour afterwards), the more you take at once, the less bio-available it becomes (someone correct me if I'm wrong here). I would recommend for example magnesium L-lactate dihydrate sustained release formulations instead of magnesium oxide. Magnesium oxide isn't absorbed in the stomach but instead in the small intestine. There's a catch though, magnesium oxide has to be converted to a more water soluble form by the stomach acids for it to be absorbed in the small intestine. This is not the fastest process, so the longer it stays in your stomach, the higher will the amount of absorbed magnesium be. I'd argue that generally you want steady amounts of these nutrients in your bloodstream, instead of having the amounts fluctuate a lot by taking megadoses of supplements every now and then instead of taking small amounts at regular intervals.
Could be that they were just bad pills. Maybe they were from a good and reputable batch yeah, but perhaps these individuals were improperly stored that resulted in some degradation of the active ingredients. Who knows if they even were 100% mdma but instead had an ingredient in them that didn't really work well with magnesium thus making you only feel partial effects. Plenty of things to speculate on *shrugs*.
