I can make MINE stay up - I THINK the secret is to HOLD THE BUTTON DOWN until the spoiler is up all the way, and KEEP HOLDING IT DOWN TILL YOU GET A BEEP!

That BEEP seems to indicate that it is in full manual mode and will ignore the fact that you have slowed down.

I keep mine up quite often - when I head for church I often lock it up and once I get there, it stays up!

WAIT FOR THE BEEP before you release the button!