I don't know if this is pure coincidence or not, but I just finished camping the NM after about four hours. During that entire time, I was the only thing around the spawn point, and each time it only reported that "the sprout is looking better." At the end of the final waiting period, however, rarabs and sylvanestre suddenly started spawning around me -- like I said, I was the only thing there before that. When I tried the water that time, the NM spawned.
Coincidence? -- Kireinekonyanya (Fairy)