I'm running into some issues and I have been scratching my head for some time, so I thought I would give you guys a go.
We are using the compute module 3 on a custom board with custom raspbian based OS. All works, but sometimes we run into 'faulthy' cm3's while trying to boot. We flash our OS image to the boards with Etcher, all is good and verification says all is fine. When we try to boot the module though it does not do anything. The activity LED just stays on and there is no screen. The strange thing is that this occurs maybe 1 in 20 compute modules. Others work just fine.
I thought it might be something with the flash chips having a different size, so the image and partitions have been resized to create additional unused space on the eMMC. Unfortunately this did not help.
The compute modules currently on my desk with these issues are all made in China.. not sure that would be the cause because others work just fine, but might be worthwhile mentioning.
The strange thing is, I can flash these modules with a clean raspbian and they work... just our custom image won't work on them. But when I use a different compute module I can flash our image and all works, just on these specific versions it does not do anything.