Structures in Nether-type dimension are very rare even when set to high spawn probability

Alright, so I've come across an issue with getting custom looking tree things to generate using structures in a Nether-type dimension's biomes. Even if I set the spawn probability to super high (say, 800,000), it very rarely generates them. I'm sure this could be fixed using custom code (or editing the code of the structure spawn), but I'm not sure where to begin or what to change.