Can not excute steps in the process model

hi all:

I kicked off a conversation of pip7B1 process model.
When the “wait for ACK” step received the ACK from other side,the next step “verify ACK” could not be excuted.And following message was given:
“can not excute the step N13 because the process was terminated already.”
Who can tell me why???
I waiting for your help! thank you very much!