Mangrove Trees Won't Grow

I'm having trouble manually growing Mangrove trees. I have a big, clear space of mud, plenty of room for them to grow, but about 75% of the time they just refuse to grow, even if I spam the Propagule with bonemeal for a full minute. I've tried planting them on all different kinds of dirt, above and below water, different light levels, different biomes, and none of that seems to make any difference.