Assuming you don't have any loss of compression due to the continued use of combustible substances sprayed into the air intake, I would connect a scantool and read fuel rail pressure when cranking the engine. FRP needs to achieve around 10,000KPa (100 Bar or 1,470psi) before the fuel injectors are activated. If the FRP takes a while to reach this then fuel could be draining back to the tank over a period of time, or the pressure relief valve, high pressure pump or injector/s could be faulty. A leak-off test would confirm this.

Direct injection diesel engines which includes all common rails do not need, or employ, glow plugs for cold starting. Their purpose is to reduce noise and improve emissions when the engine is running cold. They only switch on for pre-heating when the outside air temperature is near freezing.