migrating to sql native client causes question marks on db

Question

we have migrated to native sql server 10. since then everything that pass throw the c++ code to the data base with unicode character such as Hebrew turned into question mark. before the migration everything worked fine. any idea why this is happens?

thanks

EDIT: Connection string builder:

 res =  _T("Provider=") + sprovider +
            _T(";Data Source=") + data_source +
            _T(";Initial Catalog=") + initial_catalog +
            _T(";User Id=") + username +
            _T(";Password=") + password;

res += _T(";DataTypeCompatibility=80");

as for other places in the code we use it is used in a very wide context. it is very hard to show examples with a system as big as this. any way i can a sure you it worked fine before with unicode , this are nor a new tables