Not knowing the full details of your situation, it's hard to say why the Client won't accept the prequalified joint.
However, I can say that your previous 6GR PQR is not valid for any materials other than Group I. see AWS D1.1- 2000- 4.7.3, Table 4.8, and Table 4.6 1).
In order to find the information necessary to qualify the WPS you need, see 4.12.
Good Luck,
Tim