The diode symbol has two parts: the triangle and the stripe. If a positive voltage is applied to the triangle side of the diode, the triangle is pointing the direction the current will flow meaning it will pass through the diode to the other side. If the positive voltage is applied to the stripe side of the diode, the stripe acts like a road block. It will not allow current to pass through to the other side. There is a small voltage drop across the diode (approx .7 volts). So make sure your source voltage is good. Time to go back to school.