Your soil could be a bit rich for natives, especially if you've added any fertiliser. I think the first thing I'd do is to take a goodly sample of your soil to a nursery and have it tested. If the ph. is above average most natives will fail.

Is the average ph '0'? I've tested the ph in the soil but I think it's time to ask the professionals! Thank you, I"ll do that.

No. The average is 7 - the 'neutral' point, neither alkaline nor 'acid'. Some natives will tolerate slightly alkaline but most prefer neutral.
B.

If you've dug a hole in heavy clay your problem could be drainage. The roots could be rotting from sitting in stagnating water . Root rot from too much water can have the same effect as too little water. Mine seem reasonably tough, but not real fast growing.

Remembering that xanthostemon chrysanthus is a rainforest plant, you might have a double problem there.
First, though it's regarded as not having an invasive root system, it does tend to have a fine mat of surface roots, adn I think in this case you don't have a deep enough drip line, and there is too much competition from the grass.
If you want a neat line for mowing rather than a free-form, I'd just get something new for a surround, widen it to about 3 times its present width and gently remove the grass, then mulch the ground.
Secondly though they will take full sun when fully grown, and stay a reasonable size instead of soaring for the sun as they would if competing with trees, you might find that spot is just too hot for it at its present time. Factoring in reflected heat from the grass and the edging, and the fact that, as I say, its surface roots may be suffering in hot weather while the lawn sucks up the moisture. Maybe a little shade enclosure for summer while you improve its conditions?