[R&F] Can't build Great Wall on most tiles

[VCrakeV]

Playing as china, masonry unlocked. I have Quo's Combined Tweaks, but that should only make it easier to build the Great Wall (it's supposed to allow for building it outside of borders). Anyway, I can't build the Great Wall on most tiles, but some I can, for some bizarre reason. I'll post a screenshot soon, but the odd thing is that one tile is a plains hill, no forest or jungle, no resource (unless something I haven't discovered yet, like niter), nothing. On this tile, I can't build the great wall (and it is in my borders). Yet, on a grassland hill not far from it, I can; there are also a few desert hills where I can build it, and a few where I can't. Can I not build it where there is a resource I've yet to discover?

edit: as you can see, I can build in some places I should, but I can't in similar tiles that I should be able to.