General Motors Co Chief Executive Mary Barra’s strategy of rebuilding the Cadillac brand has run into twin roadblocks: the carmaker can’t persuade enough American sedan buyers to switch from foreign competitors such as BMW or Mercedes, and it can’t make enough of its Escalade SUVs to satisfy domestic demand. By contrast, its dealers are offering discounts of thousands of dollars on the ATS sedan, intended to compete with the BMW 3 series, and on the CTS sedan, meant to rival the Mercedes E-class.