Feijoa help... please!

I've been trying to propagate a small feijoa tree I have (usually grown in New Zealand) that's in a pot. I'm in the UK. I've read online supposedly the best methods to propagate this tree and have attempted them the last couple of years, however, after trying at least 50 cuttings with each of the following methods and getting zero results, I've had zero success, so I need some help.

I've tried cuttings with and without rooting hormone, cuttings with fresh green shoots, cuttings with older harder wood and cuttings from 1 year of growth. I've tried cuttings stripping the lower inch outer layer back, and without, trying cuttings with and without rooting hormone in soil, sand, wood chips or just in a glass jar with water. None have produced any roots at all. All attempts were made between from April-September during the main plant growing period. I've tried 3 different brands of rooting hormone in case the first and second were bad or something.

There's not been any fruit for me to grow from seed yet but the tree is only 4-5 years old so I believe it should produce something within the next couple of years, it appears like it might be my only option at this point once it does though.

Does anyone have any tips please? This is one of the only plants I'm really struggling to reproduce, and most people online say it's one of the easiest!

I have a couple of Feijoa plants where my polytunnel used to be, but I've never tried propagating them. I'm getting a new tunnel, so making some new plants as backup in case they don't transplant sounds like a great plan, thanks for the idea Bri!
According to the RHS website, apart from growing from seed, Feijoa can be propagated by semi-ripe cuttings in summer. I find these much more tricky than hardwood cuttings, as they seem much more likely to fail due to mildew rather than make roots. I usually take small tip cuttings (maybe 4in long) remove the lower leaves and maybe trim larger leaves, then stick several in a pot of damp compost, cover with a plastic bag to maintain moisture and put on the window sill. I remove the bag every day of so and spray the pot with water to try and stop the cuttings from drying out. I did manage to get some Aronia cuttings to take last year by just sticking them in the ground in the shade and hoping our damp climate would do the rest! But more than half didn't make it. I wish I had better advice, and look forwards to more suggestions to improve my chances too.

I've only tried once or twice to root hardwood cuttings from our feijoas pruned in the winter...no luck. I have had a few pop up at random from seed, most likely bird-sourced, and one of those produces the biggest fruit of all the ones on the property. Given how ruthlessly I prune the trees we have, it would be nice to propagate them by cuttings but with such an excess I haven't been motivated.

I've seen references to summer semi-hardwood cuttings being more likely to root, as Nancy mentions, but airflow is more tricky to manage. This seems to be a theme with lots of plants in the myrtle family. If only they had a profusion of aerial roots like their cousin the pōhutukawa....

Thanks folks. I forgot to mention that I tried a batch covered by polythene bags to keep the moisture in also with no joy. I guess I'll have to wait for a few seeds before I try again.

No experience propagating feijoa and the deer and 'roos have eaten my plant down to a stick;  however, I've recently had success with a caper plant - notoriously difficult to do, apparently.  I used a method shared by a friend - take your cutting, use a sharp knife to cut a short slit in the bottom,  into this slit, insert a grain of rice - I used long grain Jasmine, but I expect that doesn't matter!    Place into your soil in a pot to start with to keep an eye on it.  Water if necessary (low rainfall area for me).  It may help to reduce the number of leaves, to reduce stress on the cutting.
Sounds a bit like witchcraft . . . but so far so good.    Now I'm wondering if it works with other "woody" cuttings.  

The only way I've ever seen feijoa propagate is by layering, where a low branch ends up buried in the mulch layer and gradually takes root, sometimes over a year or more; and can then be cut free from the main plant and carefully transplanted.