Ok thanks for the response.
On the decompression knob use to hopefully help with starting:
Turn key on like you normally would. Turn switch to glow plug position and hold for the 10 to 15 seconds like normal. At the last few seconds of that (or anytime before cranking really) pull the decompression knob out and then go to the crank position of the switch. The engine will spin hopefully faster than it has been. When the engine is at top spinning speed after a couple of seconds then push the decompression knob back in and the engine should fire off. But the decompression knob must be fully in.
Don’t use the decompression knob as a kill switch unless you have a problem with the engine being in a runaway. There should be a fuel shutdown lever that stops the engine.
If the starter still doesn’t start it, then you may want to get it rebuilt or replace it with a newer type (gear reduction) that will spin the engine more quickly.
This is all assuming that it is working properly. If this process doesn’t help then this site is full of questions and answers on glow plugs and glow plug indicators.
Come back with results when you can.