We were playing on a ribbon world + Bob's and I found that certain resources were no where to be found. I understand that some items will be difficult to get but we went pretty far and nothing was spawning.

I even entered god mode and ran around to check for it.

I would need to validate this but I think that ribbon world that is smaller then region size will confuse ore spawning algorithm. By default thats 7 chunks = 210 tiles. Since ribbon world is symetrical around axis and RSO uses this axis as region divider also you would need to have world height twice the region size to work well. It might work decently with height about 1.5 times but lower you get more resources will end up in the void and collision avoidance will not be able to cope.
You can try reducing region size and see if it helps - it's not well supported on already existing map but it will work. You migth see double ore spawns if you reduce it on running game.

Hey I just to follow up to see about this. One suggestion to add a custom world that is at least 210 height (RSO Ribbon world) as the default is 128.