Cool
You might need to look out for a fastboot option in phone Settings menu (somewhere) on some Sense ROMs. If this is ticked phone doesn't Power Off as normal - think it enters a kind of hibernate state, always unticked it straight away when i've spotted it in Sense ROMs - so this might be what's possibly interfering with trying to get into Hboot using vol down/power button.
You think phone's powered off but it isn't so all you're actually doing is waking it up. Worth having a look, unticking this, power off, then re-try the vol down/power to get into Hboot