The plants in your picture don’t look like feijoa/pineapple guava. The leaves are pointed instead of rounded at the tip, and they also look smooth rather than fuzzy on the undersides. Your plants look more like strawberry guava (Psidium cattleyanum) to me.
My Fejoia looks like the @ncdabbler plant too. I’ve also read (somewhere on this forum) that they are challenging to graft, ESPECIALLY the older ones like I have. That said, I have yet to get flowers or fruit on my poor potted plants so air layers and grafting may be in my future. You have to at least try, right?
I had no trouble grafting two feijoa cultivars using cleft grafts with dormant first-year hardened wood, performed in early spring. Another one that I tried later in spring had total failure, though. I’m planning to try again this spring for the failed one, not sure if it was timing related or if that particular plant is just tougher to graft.