If you've got the Cluster HAT connected and you're having problems with one or more Pi Zeros not being visible after booting with an SD card please check through these steps and report any issues on the forum.

1) After powering on the Controller Pi is the PWR LED on the HAT lit up, and after Raspbian boots does the ALERT LED go from dim to off?

- If not check the HAT is plugged into the Controller Pi properly.

2) When you run "clusterctrl status" does it say "clusterhat:1" ?

- If it doesn't check the Cluster HAT is pressed onto the Controller Pi GPIO pins properly (you will need to reboot the Controller Pi before it will be detected).

3) Check the USB hub is visible, run "clusterctrl hub off", "lsusb -t" (save the output), "clusterctrl hub on", run "lsusb -t" again and compare.

- If you don't see the 4 port HUB check the USB cable (try another cable ensuring the cable supports data as some are power only).

4) With the power off to the Pi Zeros "clusterctrl off", remove their SD cards, run "sudo service clusterctrl-rpiboot stop", "clusterctrl on" and then wait a minute.

5) Do the P1-P4 LED (on the Cluser HAT) come on (you may need to run "clusterctrl led on" if you've turned the LED off)?

- If not then probably an issue with the HAT.

6) Do you see four 12M USB devices hanging off the USB hub detected above?

- If you do it's a problem with either the SD card itself or the Raspbian installation on the SD cards, please try to write the image again and/or use a different SD card if you have one available.
- If you don't them make sure you've pressed the Pi Zeros onto the connectors far enough (if new they will normally click), you should be able to see the hooks through the holes in the connectors on the Pi Zero if it's plugged in far enough.

7) Power off the Pi Zeros "clusterctrl off", replace the SD cards, and power on "clusterctrl on" and wait a minute or two (the initial boot will take longer as it will resize the image to the size of the SD card).

8) Does "ls -l /dev/ttyACM* /dev/ttypi*" show the USB Gadget Serial ports and links?

9) Does "sudo ifconfig -a" show the ethpi1 to ethpi4 interfaces?

- If not, do you have any "renameX" interfaces? If so then it's likely you have a MAC address conflict (the MAC should correspond to the Px number so P1 should use "00:22:82:ff:fe:01", P2 "00:22:82:ff:fe:02", etc.). If you have a "renameX" interface check if the MAC address is the same as one of the ethpiX interfaces, if so you will need to either reconfigure the SD card (see Intermediate Setup section) or re-image the SD card with the correct image.

 If you're still having problems please post details of your setup to the forum - Cluster HAT model, Controller Pi Model, filenames of the image used on the controller (cnat/cbridge at a minimum) and Pi Zeros image filenames,  how you're connecting to the internet (wifi/ethernet/etc) - basically please explain it like me like I don't know your setup :).