HC-SR04 pulse at multiple angles
I doubt you have misunderstood the way ultra sound works.  You send a pulse and then go quiet waiting for a reply.  The time it takes to send the pulse is the minimum distance you can detect things at.  

If you are not getting returns left and right then it could be there is nothing there or turning the head is breaking a connection.  

Firstly jack the vehicle up off the floor so it doesn't go anywhere.
Place a solid object (book) in front of the vehicle and make sure it detects it.
Now place the same object to the right of the vehicle, do it detect it?
Stop the US sensor when it is looking right (by killing the power) and check that it is looking horizontally.  Check the connections, are they being pulled loose?

Your code looks fine, you US works forwards so I think it is a mechanical issue with your car.