There are a number of questions this raises:
1. How are the cameras configured to stream? RTP over TCP, RTP over UDP, Multicast, other? If the cameras are using TCP over wireless, you can get delay due to acknowledgement round trip from server. Even if router to router ping round trip is low, this could still be trouble. And some camera/server combinations have issues even using UDP where the application layer is responsible retransmissions.
2. How much interference is there? Have you coorelated noise floor, BER, etc to video performance. Could be that you are just not seeing some interference when you are checking the network. Turn on NTP and make sure that cameras and routers all pull time from the same source. Then turn on syslogd with debug logging from your router and cameras. Then compare the two.
3. Have you tested this specific wireless link with another camera brand of same resolution and frame rate?
4. Can you run dynamic frequency selection or use a script to change frequencies when BER hits a certain threshold?
With wireless, in heavily congested areas, you cannot always find easy solutions. Something that works in the parking lot where there is little interference may not work in the center of the shopping center where every shop has an AP.