For hard to transfect cells, like neuronal or iPSCs, electroporation is a good choice. Try to transfect your cells using 20micrograms of DNA (or RNA) in a 6-well format (5-10million cells). Electroporation settings: 240V, 500uF, resistance: ∞ , 1 pulse. Good luck!
For your cells, I can recommend another transfection technology call Magnetofection especially the NeuroMag reagents with has shown a excellent efficiency in a variety of neuronal cell lines and also primary cells