Are these systems far appart or in same building etc?
If its local LAN, I'd try disabling the firewall of the system that is resisting the remote connection to see if that works. If that works then you know you are dealing with having to add an firewall exception. If it doesnt work then its something else.
If its a site to site RDP connection, if these sites are within driving distance, bring these 2 systems together on a local LAN and see if you can connect to each one via RDP. If you can then you can move forward with checking router port forwarding configurations etc, if it doesnt solve the problem, try shutting off firewall and seeing if that resolves issue. If it does resolve issue then you need a firewall exception, otherwise if it doesnt its something different.
I assume that both are on the same workgroup or joined to same domain, and the RDP has been properly configured on each system that is accepting RDP requests?