when using a half bridge constructed from N-channel MOSFETs, the top gate voltage has to be above the source voltage by a margin greater than the gate threshold. or something like that, @hurz knows the details.
you could just use a P channel on the topside. if you're rich.
the next best thing is to use a 16V supply to drive the top gate above 12V.
failing that, you could try forcing the gate node into very high impedance and then pulse into it. like this circuit attempts to do.
