RosettaNet process same step executed more times in one process instance.

Our webMethods IS server is 9.12, and clustered with two IS server.
RN package version is
I used designer import one action process, sent out one testing RosettaNet message, everything work fine.
But, we found that we will have many ‘Send External Document’ steps during 10 seconds.
According to this RN one action process design, after ‘Send External Document’, it should be flow to ‘Wait for Ack’.
But, we found the other ‘Send External Document’ appear in 10 seconds.
And, according to this RN one action process design, if ‘Send External Document’ retry 4 times, it will flow to ‘startNOF’.
So, this one action process always flow to ‘startNOF’ step.

We think it maybe Process Engine Clustered cause this issue, is it right?
Do anyone know how to let this process flow one step by one step?