Canbt trigger the next step in RosettaNet 3A4 process

I implemented RosettaNet 3A4 proccess with webMethods, using https protocol.
After the buyer side started the process, I, as a seller, could get PO request from the buyer side and start sending Receipt ACK to the buyer side. The buyer side can get the ACK correctly, but Our side process suspend on Send Receipt ACK step (The Send Receipt ACK step status is stated) and can¡¯t find error on my server log. Please help me for this question. I used webMethods601, and TN profile configured, Modeler configured correctly. Thanks in advance.