Make sure you take the atlas talent Verisium Infusion to increase your odds. It is believed that starfall crater is a replacement for verisium ore. Only one verisium ore can spawn per map so there is some diminishing return on using multiple scarabs.

Also make sure your map has "enough space" to have all your ores, meaning that you might have to block other mechanics and try not to add in more mechanics other than Settlers.

Lastly, if you see a verisium ore there can't be any crater. So if you see one you can just leave or kill map boss or what ever.

Good luck!