It could be several reasons:
- a host based firewall
- null account password
- ISP filtering at the application layer
- incompatible RDP client
- trying RDP from internal host to WAN IP / most routers won't let internal traffic back in.
How did you verify port forwarding is properly functioning if you cannot connect?
Either follow Grumpy's suggestion, try VNC, pay for Radmin, or post more details.