problem with composite

[andrearrd]

Hello everyone, I am using ABAQUS/Standard (6.2) to model a stiffened composite plate under buckling. I create the model in CAE, and then asssume that it is made of a linear elastic material and the buckling eigenvalue analysis works. When i then edit the input file manually, replacing the various section definitions with Shell sections and corresponding composite definitions (*orientation, composite etc, and define the material as a lamina) the input file gives an error saying that "one or more elements have not been defined or are zero". The line corresponding to the error is the *element line which i have not edited and was created in CAE (and gave no problems when i used the linear elastic material). Any ideas on what could be wrong?
Thanks
andrearrd