Received fatal alert: certificate_unknown

Dear 

I have the following Scenario 

one web service A working two-way SSL ,I already create a certificate portecle application.

one client B send a request to web service A  I  already create a certificateOpenSSL and i used SOAPUI to send request.

when I try to send a request client B to A  it returns following error Received fatal alert: certificate_unknown.

when I used the cretificate from particle  for A and B it's working !