Motor controller - transistor taking up 3/4th of the voltage
You're not turning Q5 on properly. It needs a certain Vgs to turn on fully. Measure Vgs on Q8 which works and arrange for the same Vgs on Q5. (Start by working out the source voltage on Q5 when it is fully on).

This usually means arranging for Q5 to be driven from a different signal (generated by a level shifter). Which means a supply voltage greater than 12V to power the gate drive.

Or using a different FET for Q5 (usually a PMOS with source connected to 12V, drain connected to load, and pulling the drive voltage down from 12V.