u/TinfoilHyena

I'm playing in the national dex format, and have a toxic orb and the ability poison heal on my breloom. I tried to use leech seed as well, but it doesn't seem to work. The opposing pokemon will record as seeded, but the leech seed does not sap their health, and breloom only gets HP from the poison heal. Do they not stack? I can't find anything in the move descriptions that would prevent this unless I'm missing someothing.