APPIAN Web Service Authentication Issue

I have created a Process Model which I expose as a Web Service. When I try to call it from an external application, something like SOAP UI, I am getting the following error. I am providing a valid credential as input. Even I tried to login using that credential. Any suggestion on why the authentication issue is occurring?

<?xml version="1.0" encoding="UTF-8"?><SOAP-ENV:Envelope xmlns:SOAP-ENV="">
<faultstring>Authentication failed</faultstring>


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