Hi Dongkai
This might be because of filter options. I understand that you run the same DTP with same filter conditions.
But there is a filter for debugging alone which might have been selected by someone else or by you in earlier cases for some reason.
Before clicking on Simulate, check Expert Mode option next to it. and then click Simulate.
You will get the filter screen for debugging which is not the same as your normal DTP filter.
Try and let us know if it works
Regards
Karthik