Thanks for the reply.
I just swapped the two motors. Motor B again (physically the other motor) rotates only when direction is set to HIGH.
Led on B is not flashing and measuring the voltage on the motor B output is 0.
I also tried to do some more measurements on the motor shield.
I disconnected the motor power supply and measured the voltage difference between pins on L298
using the schematic provided for the shield (Schematic)
When setting speed to 250 the voltage between DIRA and DIRA# is 4.7 or -4.7 depending on direction
However for motor b DIRB and DIRB# is 4.7 or 0
So i think the nor gate is between brake and direction pins for motor B is broken...