You must make sure the voltage on the gate is sufficient to turn the FET on fully. If it is only partially turned on the LED will illuminate but the voltage across the FET will not be low enough to prvide a LOW for the following FET.
Measure the voltage across the FET to find this.
Not really. You need to turn each FET on fully to get the circuit to work.
Measure the voltage across the FET to find this.
I think you need to balance the load currents on all stages
Not really. You need to turn each FET on fully to get the circuit to work.