Gardenia sudden wilting

Hi all,

Gardenias that we planted approximately 3 months ago were going really well, then the pictured Gardenia started to suddenly wilt, on its way to the afterlife.

The plants are watered daily as they’re in a sunny position, with fertiliser applied on a bi weekly occasion. We’ve previously found big grubs in the soil (and beetles) so used a beetle fertiliser solution which did work.

We’d tried adding mulch previously but this trapped a lot of heat and killed the previous Gardenias.

Any advice on what could have caused this would be appreciated.