On Win10 Luban version 3.12.2 I try to generate code for https://www.thingiverse.com/thing:4641906 (last I tried the hexagon, but it looks like they all fail about the same way) but after clicking Generate G-Code, only 2 “triangles” show up in the middle, whereas the model had many before I clicked Generate G-Code.
Start by making sure the smallest feature is larger than the smallest printable size, which is about 0.5mm. The thingiverse page indicates the models were designed to be ~10cm, and have thin details in them, so you could easily check that by scaling up up about 50cm or so and making sure it generates then.
If that fails the model is probably non-manifold (term for “it has holes”) or something else in which case it will need repair.
I have had this happen with printing in the past. The features were smaller than the nozzle diameter. I was doing the designing so could change the size and the problem was fixed. This was back in the Snapmakerjs days