Something wrong with digest in ack.

Hi everyone:
I’m new in webMethods and my English is not very well.

That is my question:
When I send a 3A4R (3A9R or other PIPs with R) message to partner and he reply the ack to me, in TN the ack is correct, but in MWS the process is error, Verify Ack error, the error message is “Incoming Message Digest is not the same as the Original Message Digest!..”

Thanks a lot.