I'm a bit of a novice to plants so I need some help.

I got this cordyline australis almost three months ago. It was completely green when I bought it and seemed pretty healthy. I planted it in a fairly small pot (too small, I now realise) in a south-facing location so it gets sunlight all day, which I read the green-leafed ones are supposed to be ok with.

A couple weeks after I got it home, it started getting some yellow patches and brown tips. The pot had good drainage and the soil seemed to drain all right. A horticulturist at a garden centre said I should treat with Roseclear and repot to something bigger and give it a fertiliser.

I did that and drilled extra holes into the pot but almost a couple months later, it still seems to be going downhill, with much of the yellow turning to brown and other leaves drying out and crisping. I recently removed a lot of lower leaves that had dried out. I don't water it very often for fear of overwatering and the last water it got would have been the last time it rained, maybe a week ago, but I stuck my finger in the soil today and it's still a bit moist so it's hardly thirsty?

When I repotted it, I left the rootball intact as it was pretty solid. Should I repot again and try to break apart the rootball and entirely replace the soil into something even better draining? I'd presumed the soil it came in was suitable and didn't need replacing but was I wrong?

Also, should I remove the layer of bark? It's barely an inch deep.

Any help or advice would be appreciated!

Cordyline australis help