Hi Fadil
Does the 18 channel WDM system you have designed work well for a short distance and for 10 Gb/s data rate ?
The performance of the system is expected to change if you are increasing your data rate to 20 Gb/s and simulating it for long transmission reach.You will have to change different system parameters at the transmitter,reciever and in the fiber connecting the two.
Try increasing the launch power of all the 18 sources.
Check if you have properly defined the channels for your WDM MUX and DEMUX.
Use a spectrum analyser to check the power level of all the signals at different points in your design.
These are a few points you can use to trouble shoot your system
see if it helps
Regards