There could be many reasons, but the most common are:
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The port is not open on the destination machine.
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The port is open on the destination machine, but its backlog of pending connections is full.
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A firewall between the client and server is blocking access (also check local firewalls).
After checking for firewalls and that the port is open, use telnet to connect to the ip/port to test connectivity. This removes any potential issues from your application.