Does appear odd that your developer key worked and now doesn't.
Does it make any difference to try with and without the power adapter being plugged in?
Holding down the "check marked" game key on the right while initially powering on will give you a few more seconds to press or tap the "esc" key.
Could the esc key be not working? (Once you get to a prompt you could test-all and try the keyboard thoroughly.)
Sometimes the esc sequence is just finicky...reboot a few times and see if timing works.
Finally, have you disabled security per http://wiki.laptop.org/go/Activation_and_developer_keys