I obtained 3 Nether Warts from a Nether Fortress, and I've been attempting to manually farm them for hours ever since, but there are major issues/preventions hampering ... the nether wart 1) does not grow except for the original sources; 2) only yield 1 even if harvested at MAX graphic; and 3) neither time nor other external forces (light level (dark or bright), lava nearby, or water nearby) matters not.

More than 3 hours ago, I collected 3 Nether Warts from chests in a Nether Fortress and all my efforts ever since to farm them atop Soul Sand (in the Overworld and Nether) have 1) netted me just 2 more, but now they all either 1) do not grow, or 2) do not yield more than 1 when harvested.