Combinations - Why does MDMA not work at all on SWIM?

[Moonage Daydream]

As has been said elsewhere SWIM has been doing amphetamines for four years, not every day but very often (say 3-5 times a week)
She has noticed that when she takes MDMA (even powder/crystals which work very well on all the others she is with) it either fails to work completely or produces a tiny effect.
For this reason obvious SWIM doesn't take it anymore as it would be a waste of time and money (and her arts job pays v stingily), and in the name of damage limitation because at least then SWIM is just putting one relatively simple chemical into her body. Maybe it's worth mentioning that, unusually for all others into speed that she knows, SWIM does not smoke (ANYTHING) and drinks very moderately. So only bad habit is speed.
By and large SWIM is rather bored by MDMA anyway, which is associated with her boredom with the part of the club scene she used to be involved in (more gigs and genuinely interesting electro/experimental stuff these days)
Having said that it would be nice to be able to do it every now and then as she did enjoy it on occasion. SWIM can't do psychedelics even though she is interested as she is too fundamentally nervous and over-thinks like mad. These are inborn character traits which are of course enhanced by amphetamines. Swim dislikes downers and is scaredy cat about ketamine and the like, and is either missing something or (only in SWIMs subjective opinion) cocaine seems to be the most over-rated drug in the world after alcohol. So options are limited if SWIM ever feels the occasional need to erm, vary her diet.
Yeah SWIM knows the obvious answer is to quit amphetamines but this would be exceedingly difficult, though SWIM knows that she will have to eventually.
Anyway thanks and sorry to babble on as ever. Ah well, tie her to the chair and gaffer tape her mouth, that should do it ha ha!