I have GWX. I know there is a random element but I've had this happen with two lg merchies now. Both times Ifired two steamers and got two hits with virtually no effect both times it took multiple hits and i had to linger in the target area much longer then I'd prefer.
Next time I'm going to try to space out my shots a bit more as suggested above.
But no way I'm letting an 11000 ton target escape

It just seemed strange that any merchant could take 4 torpedo hits and still last a long time. But thats the War for you.