Airport Express won't extend network

Hi all,

I had a look on the forum but couldn't see anything similar to my problem, so:

I have a moderately complex network setup at home:
- a Time Capsule 3TB with ac
- a Time Capsule 1TB with n, connected via power link to the other TC
- a newer Airport Express (the flat model with n)
- an older AE (the model with a/b/g only)

I got the new TC about three months ago, and it took a few days to get the system setup but seemed to be working. About two weeks ago, both my MBP and my wife's MBA started having problems where they couldn't connect to the internet over wifi, although all out iOS devices could use this network successfully.

After some diagnostic fiddling it seemed like the two TCs weren't playing well together, so I switched off the older TC which wasn't really adding a lot to the setup, and this has helped the Macs tremendously. I did this yesterday.

Now the problem: as of this morning my newer AE is unable to see my network any more. It would just flash an amber LED. Using AirPort Utility, if I was closer to my other kit and the device attempted to access via either the TC or older AE then the new AE appears with a yellow warning triangle. If I am closer to the newer AE then it attempts to access the network from there and I get an amber circle with the warning that the device has no internet connection and the other two devices get the yellow warning triangle. It is fairly clear that the new AE is not attached to the network.

I attempted a power cycle, with no effect. Then I did a factory reset (holding the pin switch until I got a rapidly flashing amber LED). That allowed me to find the AE using its default name. When I attempt to set it up to extend my network, it takes the setting and restarts, but then instantly goes back to a flashing amber light showing no network connection. I have tried this with the device sat next to my TC, so the wifi link should definitely be there. I can physically link the devices for diagnostic purposes, and in that case I can get a green LED, but as soon as I remove the cable I'm back to flashing amber.

Any further suggestions? Is there a way I can re-flash the AE somehow with a downloaded file from Apple?

All ideas appreciated!

ETA:
I can ping the AE when it is attached via ethernet, but as soon as I disconnect the physical link I get 100% packet loss.
I have downgraded the firmware; the problem still exists. I upgraded back to the latest firmware, still exists!

Airport Express won't extend network (Fixed)

OK, I've managed to identify the problem, and fix it, but I have no idea how it came about!

After talking over the problem with a tech-y friend I remembered that I have a wifi monitor, so I could verify whether or not the AE was actually broadcasting. I fired this up and could see the problematic AE, and the apparent cause of the problem.

I live in a fairly dense area and there are several wifi networks which overlap mine. Because of this I have manually moved my wifi channel to 3 (the default tends to be 6, and a couple of neighbours are using this channel already). Looking at the problematic AE, it was using channel 6. For some reason, it had forgotten it should be on 3.

I changed the network back to 6 temporarily, and normal comms were restored to the AE. Then I changed the network back to 3 and the AE took the change, staying in sync.

Problem fixed.