Linh,
I'm actually working through the exact same thing right now - I've just opened up a case.
It seems kind of intermittent, but here are a few things that I've found help in the past:
1) Make sure you're specifying the actionCode="01" for creation of objects/nodes
2) Make sure the "TestDataIndicator" is false
3) Make sure any "ListTransmissionCompleteIndicator" is "true".
4) Make sure you're following your XSD exactly.
5) Start by testing with a small file size - i find 100 Kb max is a good start - you can increase it once you have things working.
Other than that, i don't know of any other diagnostic options - i'm hoping that support is able to suggest some.