How to turn on LP Delta again after turning off?!

I've got a LP Delta with Windows pre-installed and have come across an odd problem. The unit starts up fine when power is connected and shuts down as expected after the power button is later pressed. The problem I have though is that it is then very difficult to turn the unit back on via the power button. I've tried long presses (more than several seconds) and short presses of the power button, but the only thing that seems to intermittently work is a combination of a long press followed by a second short press. This is not very practical and it only works one time out of ~5!

Am I missing something really obvious here? Note that I've also tried connecting a remote power switch to the SW header but the issue remains.