@nerky,

The brick wall will definitely be the factor in this situation as it will absorb to block most signals that try to pass through it, especially for the 5Ghz frequency. The ability to manually select the source of the signal for a deco node is currently being developed and has only been released to a few models so far. A fix could be to potentially move the decos in such a way that the signal will have to travel a lesser distance between nodes and may have better luck going through the brick. 

Also, if the devices are all daisy-chained together in a line, meaning the signal is being forwarded by an extra deco on the way back to the main, there is a much higher chance of the signal being degraded or slowed. Connecting some of the decos with an Ethernet Cable would serve to fix this behavior.