As the city grows I encountered this issue. There are more than enough mines, and many are sitting idle and full while certain city blocks end up starving for resources (iron in this case). When I check on the buildings they aren’t even showing material in route, or minimal material (4 units). I’ve seen other posts about this and the solution was distributed mines, which I have… so I’m kinda stumped on this.
i have noticed this too and it seems to happen more with trains in the supply line. also oddly i have removed the problem building and rebuilt it and that has fixed a few of them.
I’ve had this in every play through also. Destroying a produciton building and rebuilding has solved it most cases (but not for long). Problem persists with primary road or rail networking in equal measure. Have tried zoning and couldn’t get that to solve the issue either.