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="schemas.xmlsoap.org/.../">
<SOAP-ENV:Header/>
<SOAP-ENV:Body>
<SOAP-ENV:Fault>
<faultcode>SOAP-ENV:Sender</faultcode>
<faultstring>Authentication failed</faultstring>
</SOAP-ENV:Fault>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

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