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If the channels are running at that slow a rate, small wonder if you get no pictures - there would be no time for one frame to 'assemble itself' fully, before another frame started and caused it to be dumped, thus no picture and no video, as I interpret what you've said.
The question is, why run at such a slow rate?
Perhaps your have switched operation of your network to Wireless B, which is rather slower. That's something to check on anyway: you should be running on Wireless G, or Wireless N.
Interpretation really depends upon more data about how your networking is set up ... I assumed above that you have a WiFi setup for the cameras, but for all I know it might be wired Ethernet. Let me know, in either case, what 'box' it goes through.
You might want to power Off that 'box', wait 10 minutes, then power it On again, and see if that restores operation. (But if that gets it working again, consider replacing the 'box' in question, it may have become unreliable.)