Hankj, these are my observations after 20+ years of substrates including fine gravel, pea gravel, play sand, and Flourite (Seachem's "enriched" plant substrate in black).
To the initial question of the plant rooting. There may well be other factors here, as the lack of roots and disintegrating lower stems suggests. Leaving this aside, most stem plants will root easiest in fine gravel or sand.
To the issue of types of sand. But first, the fish should determine the substrate, if any in this tank have substrate-related needs. Substrate fish such as corys should always have sand. Loaches can be sand or fine gravel, depending upon the species. Most cichlids are best with sand, as they feed from the substrate, or fine gravel or pea gravel for the larger Central American species works.
Now to sand types. I was disappointed with the black Flourite on two counts. First, plant growth was not improved over my sand and gravel tanks. Second, every spec of detritus and "stuff" showed up. Third, this is too sharp for substrate fish or cichlids.
I use play sand, now in 6 of 7 tanks, the 7th has fine dark mix gravel. These mixed sands and gravels hide detritus, and in the case of play sand the appearance is near identical to many Amazon streams, so it is natural.
Other types of sand can work, but be careful of the roughness. Play sand is made for kids to play in, so it is probably about the best you can get on this count. Now, before others jump in, I'm not saying all pool sand, etc is rough--but some of it is.