Discovered that the niobium ore patches are affected by the starting area size slider on setup. The larger your starting area size the farther away the first spawn of niobium is. Everything else being equal and niobium set to maximum frequency, with starting area at 100% the first niobium patch goin...

Ran into this same problem. I tried previewing a new map with every resource turned off except niobium and there was no niobium showing. Only way I got niobium to spawn on a map was to use the custom pymods resource generator in the mod settings.