I think I might know what the problem is - the type of rock strata of the surface layer. You can check the "ore deposits" section of the VS wiki
https://wiki.vintagestory.at/index.php/Ore_Deposits
And under ore generation - shallow ores check Cassiterite (this is the tin) - there you see it can only spawn in certain rock types - in this case its either: phyllite, slate, adesite, basalt, granite and peridotite.
So if in the area that you spawned in or travelled the surface rock layer (the one that is directly under the dirt when you dig down) is something else that the ones metnioned above like for example conglomerate, sandstone, etc - you WONT find any surface tin around - as cassiterite wont spawn in it.
I hope that helps