Why write express vi does not work while line do?

Hi,

I'm using NI9514 to control a motor drive. When I was building the VI I found that the straight line move works while the write(with either velocity or position) won't do anything. I've read about the detailed help about it and still have no idea, and there's no such example in my LabVIEW example library as it suggested. Does anyone have idea? Thanks for any help!

Best Regards