While every network might experience a hiccup where payments get out a day or two late, reputable, trustworthy networks always pay on time. If you are not getting paid by a network, you have to drop them. It's a sign of a bigger problem and continuing to run with them will leave you in a situation where you could wind up losing more in the long run.
You should always do your homework before starting to send traffic to a new network. You are, after all, trusting them with your traffic (i.e. money). Networks have to float a lot of money, and this takes careful planning to pull off properly. It's really easy to screw it up - and if the network screws it up, guess who pays? The affiliate. So don't just jump to a network because they're promising you an unrealisticly high payout, or will let you bend the rules a little, or have a shiny new toaster to give you. Look into it first. You'll be glad you did in the long run.