Procedural Foliage Generator only spawns demo objects

So I’m getting started with the procedural foliage generation tool of UE4. I have followed the tutorial in the documentation and some on YT and also tried a few things on my own.
The problem is: I get satisfying results as long as I use the trees of the OpenWorldDemo Pack they suggested. As soon as I switch to a foliage type that has a mesh of one of my own trees, it does not generate a single one. I just can’t find out why.

I already tried adjusting nearly every parameter in the FoliageType-settings, without any effect. Depending on the parameters, it also takes a while to generate this “nothing” of trees.