If you don't know what Dueling Network is. Its a Yu-Gi-Oh Site where you can make your deck and play online "For Free". This is opposed to the official YuGiOh dueling site where you have to pay to play online. Due to the massive popularity though the server limit has been reduced however.
Taken From Dueling Network:
"Reduced Server Limit
Due to excessive demand, the server has been experiencing extremely heavy load. Because of this, I am reducing the server limit to 4000 until further notice."
Meaning an average of 4000 users can only be online at a time on this site. This wouldn't be so bad if this also didn't mean you couldn't make your deck or even have a spectater. With the recent release of the Xyz set don't expect the server issues to be resolved soon.