MOSFET not turning on fully
The High Side mosfet is not turning on as you are driving the gate with the same voltage of the battery.

Assume the high side Mosfet is ON, than Vs ~ Vbat; but to have the Mosfet ON Vg must be more than Vs+Vth so more than Vbat + Vth.
In your case Vg is Vbat so Vs is just  ~ Vg-Vth = Vbat-Vth. 
The Vgs is near to Vth limit and the Mosfet is not fully turn on.
You have two solutions: 

1: replace the high side Mosfets from N channel to P channel and reverse the driving logic

2: use high side drivers to pilot the high side Mosfets. 2 half bridges are also a solution to drive both high sides and low sides.