Yes, it's most likely the inter-device delay that needs to be increased. When powering on multiple devices it sees and tries to interpret a command for a different device and misses it's own power on command. The inter-device delay will cause it to wait longer to let the previous command clear out. The clue is that help always fixes it. When you use help to fix it there are no other commands being sent. Also it tells you the configuration is correct.

Increase the inter-device delay by a large amount (3000 ms e.g.) just to make sure that's it, then go back to the original value and increase it by 100 ms until it works reliably.