Bad advice or explanations happening here.

You remove the air line, start by unthreading the nut
Discard / remove the nut & little collet / olive thing that is left on the air line (it's a brass thing, it splits in two, sometimes you pry it a bit).Slide it off the end and toss it, I believe you *never* reuse the nut or olive.
This leaves you with a naked air line. Make sure it looks good (this may have been your issue, OP)
Then , time to refit (into whatever you are sticking the air line into)
Push the line into the hole firmly (this is what you were having trouble with), and the nut is already installed on the component (you remove the white plastic bit in the end and discard it)
Pull back on the airline to verify it's "stuck" in there.
I tempt fate and snug up the nut (8mm, 7mm, 9mm, 10mm, unsure) just a little bit

I don't know what you're faced with (pics) but this is super simple. Have done this on D2, LR3, RRS, New Defender. All same thing.

This is where I learned it.

 

So I did it wrong 3x and have zero leaks

Maybe I won that powerball tonight and don’t know it yet

 

Yes, this is exactly what I attempted to do, and actually followed that video. I'm guessing it was the non-OEM block cheap-o connectors (I hope). The end of the lines are not completely smooth since they've had the retaining nut on there for 15 years, but good to know I'm not crazy and someone else has successfully done this.
I'm wary of in any way smoothing the outside of the line, I'm guessing there's a reason they are smooth and I'd imagine if I removed too much material it could prevent a good seal. I also tried putting a very light coating of silicone grease on the line to help, but that didn't seem to help much. Will report back when the OEM block arrives in a few days.

 

Yes, it is called survivorship bias. Can't hurt to buy powerball tickets though.

 

Hmm, interesting. Yep love that Arnott video haha. I changed airbags on an LR3 during the pandemic for a neighbor and I think he bought aftermarket bags, and this is exactly how we did it.

If I were you, I'd just buy whichever air line is bad and replace it from the other side and route it to where you need it. Not knowing condition, mileage or offroad exposure of your truck, I will say once these lines start getting gnarled up, they never seem to work right. They go in, but then they leak. Your problem is of course the fact that it won't even go in.

Maybe there's a bunk fitting on your component.

Anyway, these fittings are really durable and I've done many trail side repairs on LR air lines (like my L663 defender) and it just pushed back in there.

Pic of the component you're attempting to install? Might help.

 

Also btw I come to the LR3 forum because the New Defender forum is full of complete jerks... rich, mentally ill people. The rather ungentlemanly replies above in this LR3 thread surprised me.

 

Hope you get it sorted out. Let us know what ends up working for you.

 

Because when people come here for advice, and it is given, then ignore it, then break it and blame everyone but their own ineptitude.... Seriously, people work on these all the time without issue. I dont suffer fools lightly. And FYI, below is straight form VOSS on how to do these. Notice how they recommend the line is in place when installing when it comes to this particular VOSS fitting? And while they can be pushed in, the recommendation shown below is done to reduce the risk of o-ring damage because the grip ring basically damages the line when it bites it. Push in is ONLY for new lines. And even with new lines the VOSS fitting MUST be backed out otherwise the line will not go in because the grip ring will be compressed against the underside of the fitting, making the grip ring smaller than the OD of the line. But... eh... what do I know apparently.

 

So I did it right 3x and have zero leaks

Sadly, I did not win powerball tho

 

OK Disco Mike, Right it's not rocket science. If you are just undoing and reattaching a line, yes you reuse the fitting.

If you have a new component, you use or have a new fitting.

Dead simple.