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, Modeler configured correctly. Thanks in advance.

We have implemented PIP 3A4.But we are facing a typical problem at the validation level.
At the flow service pub.xml:xmlStringToXMLNode we have a validation problem.This is due to special character ‘&’ coming in at the line level which is not recognized in XML.
Please suggest any way by which the problem can be avoided without asking the end users to remove the specail characters