Hi Lan L,
First of all, thank you for deciding to use GPA!
It is difficult to say exactly what the issue is without looking at the software you are writing.
Is it possible to get a sample of the software to see if there is an issue we need to look at? Also, is it possible to get a copy of the 'About' dialog from GPA Monitor? To get this - Make sure Graphics Monitor is running -> click on the GPA Monitor icon in the task tray in Windows -> click on 'About...' -> Click on the 'Save to File...' button.