Back-to-Back PMOS high power dissipation
It's still difficult to read the schematic without a real symbol, but I think the problem is you need a negative Vgate-Vsource (i.e. Vgate &lt; Vsource) to turn on a PMOS.

You are either:


Discharging the gate-source capacitance through a resistor to turn off


OR


Pulling the gate to ground and hoping that the source voltage is positive enough relative to the gate voltage to turn on.


The MOSFET does not care what the gate voltage is (which implicitly means relative to ground). It only cares about the voltage difference between gate and source pins. This is the only voltage it can see anyways.

This can be solved by floating your gate drive so the voltage applied to the gate is relative to the source pin (and not circuit ground). For example, with a separate battery.

Also, when using a MOSFET as a switch you do not care about Vth. That is when it just BARELY starts to turn on. What you care about is the Vgs used to obtain the rated RDson (or look at the Vds-Ids curve and stay in the region where the curve is a straight line).