Depends on location too. IE: I founded a critically important city (6 resources) in a mountain pass blocking all entry to my nation for 3 other nations. Couldn't get it past level 4 - completely out of tiles.
Maybe it's not the mountains.
If a resource is in a city (unlike a resource that is in the area of influence but not touching the city) , it takes building space.
So 6 resources, each of them taking 4-6 squares, this can be the reason why you're out of building space.