There is a limitation in the EV3 firmware that makes it so that only one pair of motors can be used at a time. When the second block with ports B and D starts, it stops the motors on ports A and C and replaces them with B and D.
There is a limitation in the EV3 firmware that makes it so that only one pair of motors can be used at a time. When the second block with ports B and D starts, it stops the motors on ports A and C and replaces them with B and D.