Can't persist entity

@Override
public Order startOrder(Map<Article, Integer> article, String name, String surname, String street, String postal, String city, String email) throws ApplicationException {
    CustomerDTO customer = new CustomerDTO(name, surname, street,postal, city, email);
    try {
        em.persist(customer);
    } catch (ConstraintViolationException cvex) {
        violationException(cvex);
    } catch (Exception e) {
        sctx.setRollbackOnly();
        throw new ApplicationException("", "the order-process failed");
    }
    long customernr = customer.getCustomernr();
    return startOrder(article, customernr);
}

So I am using Java EE now for the first time. I have a distributed application and this is a method of my session bean called "ShopDAO". This is in my session bean too:

@PersistenceContext(unitName = "ShopDAOPU")
    private EntityManager em;

    @Resource
    private SessionContext sctx;

Furthermore, my session bean class is declared with the following annotations:

@DataSourceDefinition(name = "java:global/jdbc/shopdao",
        className = "org.apache.derby.jdbc.ClientXADataSource",
        serverName = "localhost",
        databaseName = "DVI-P1;create=true",
        user = "dvi",
        password = "psswrd",
        maxIdleTime = 600
)
@Stateless
@Local(ShopDAOLocal.class)

Everytime when I try to send an order, I get the message "the order-process failed". So I guess that's because the entity manager couldn't persist the customer. But why?