When i grew a few Mimosa's, i germinated them by clipping a tiny bit off the side of the seed with some nail clippers, soaked it in some freshly boiled/hot water until the seeds sank to the bottom, then i put the seed in a moist paper towel inside a plastic baggie, and when the seeds sprouted the tails i took some of those disposable plastic red cups, poked a few holes in the bottom of each one for drainage, filled them up with some garden soil, put the seeds in there about a couple inches down i think, covered them and stuck them under some fluorescent shop lights, they sprouted fine, and grew wonderfully. If i remember i didn't overwater them but i definitely didn't let them get dry, the soil was usually at least a bit moist and when the top of the soil, about a half a fingers worth of soil, was dry i'd water it. Can't remember if i misted the seedlings or not, but as the plants grew and were transplanted into actual pots, after awhile the leaves started yellowing and they needed fertilizer, so i added some fertilizer to the water, and i would spray the plants with fertilizer water as well, they loved their fertilizer for sure.
So i would say try garden soil (i think i used miracle grow or something) instead of potting soil and sand. Also are they outside, are they inside relying on sunlight from a window, or are they inside using artificial lighting? They seemed to do just fine under fluorescent lighting (i used about 3 or 4 4 foot shop lights for a few plants), but idk about window sun, and being outside they may get a little too dry especially in direct sun i would think. But yeah i would say try starting each seedling off in a plastic red cup, idk if it'd make any difference or what size pot or what not your seedlings are in but i found the plastic red cups to be quite useful for starting off seedlings, then when they've grown a bit they can be transplanted into bigger pots.
For me, i had no issue with getting them to grow, but me living in zone 8b i think it is, when the winter came, they did fine until it dipped down i think into the 20's or so and the plants froze and died off, not sure if they would've survived had i mulched around the roots or something but i hope the winters aren't going to be an issue whenever i decide to try to grow some more because really the winters where i live really don't get too cold, we only have freezing temps occasionally and when it happens it lasts a few days and then fades, so it'd suck if a mere few days of freezing temps would prevent me from growing these beauties. But the summer before the winter that year, they really took off, they loved the sun, they were growing just fine, then the winter killed them off
