Is the problem at home velvet? Or on a network at work.
Everytime a computer is turned on it contacts the dhcp server to request an ip address, subnet and gateway. The dhcp server generally provides the next availble ip address. This is more important than it sounds. The order in which the computers are turned on effect which computer gets which ip address. One day you turn computer A on first and it gets the 192.168.1.2 ip address. The next day you turn computer A is turned on second and it gets a 192.168.1.3 ip address. Port Forwarding forwards a port to a certain ip address. If that IP address changed, then that port forwarding configuration would not be setup properly. One day your port forwarding configuration would work. The day it might not. In order to avoid this problem, we need to setup a static ip address on the computer that the ports are going to be forwarded to.
In other words make sure you turn the right comp on first.