:welcome:
It is common for Glow Plugs to not function well as they age, there are a lot of easy to fix things to go wrong with the system, generally dirty and loose connections tho.
If the battery voltage is low, the cables not tight or something like that, the GP system can fail to provide the heat needed for easy starting.
As mentioned, low compression is another common problem, either from valves or cylinders/rings. Once it does start, check for "blow-by" - smoke from the engine vent, not exhaust, while under load. If it smokes a lot at warm idle, then yes the cylinders/rings are in need of overhaul.
At temperatures we have today, you should not need glow plugs to start it from sitting overnight.