Why might two containers on the same bridge network fail to reach each other even though both are running?

Common causes: they're actually on different networks (default bridge vs. a user-defined one, or different Compose projects), a firewall/security-group rule is blocking the traffic, the target's application isn't listening on the expected interface (e.g., bound to 127.0.0.1 instead of 0.0.0.0 inside the container), or the wrong port is being used.