The old method on other diesels was to partially turn the ignition on and leave the glow light indicator on the dash to turn it's self off. That meant the glow plugs were warmed up. Then you would turn on the ignition fully and the diesel engine would start.
On my current 2015 Renegade diesel the glow plug goes off immediately upon starting the engine.
If you are unsure I would recommend checking the glow plugs to ensure they are tight. Similar method to use would be comparing it to removing or tightening spark plugs.
Should that fail then I would be visiting the local garage for assistance.
Our 2016 1.6d key start wouldn't start until the fuel gauge had cycled, or there abouts, the glow light would come on and go off but it needed a second or two more, our newer 2018 1.6d pushbutton start, starts pretty much instantly. 30 seconds of flickering I can't explain though.