Custom speedtrees not visible when distant

[morcroft]

I have a strange problem with my custom speedtrees. I'm trying to generate lod for a flat region with long sightlines, so VWD trees and rocks would be really useful. I've tried both the CS and TES4qLod and get roughly the same results.

The vanilla Cottonwood is visible to really great distance: 10 cells+, but my custom speetrees aren't. I know - first reaction is "it's the billboards - go see the tutorials" but no: I tried renaming the Cottonwood's billboard to match my tree name, and that billboard shows when close but vanishes in the middle distance - while the same billboard is shown quite successfully by the real cottonwoods in the far distance.

As another little test, I've created _far.nif files for some of the big retexed rocks I'm using, and they fade in and out in the middle distance - I think further out than the custom speedtrees do, but the vanilla speedtrees are visible waaay further.

So, either:
1) there's something in the .spt
2) there's something in the CS settings
3) there's something in the LOD creation

I know this is horribly vague but does anybody have any bright ideas?