Can't connect through internal server IP, but public is fine

[Josh Wanamaker 0]

Hi, I have a strange "problem", and I'm hoping someone can explain it to me. My client system times out when it attempts to connect to my server using its internal server IP, however if put the server's public address in, the client connects immediately with no problems. I'd like to reverse this, if possible, or at least get an explanation as to why this is occurring. I've set up the proper port forwarding, there are no firewall or security restrictions active as far as I can tell, it just...times out. If I can provide any logs or anything else that may help diagnose the issue, let me know.