Does the plant wilt first? Does it wilt in sections?

P. tenuifolium are quite prone to root rots. There have been countless reports of these shrubs - often well established in the landscape - drying up and dying rapidly throughout CA over the past few years.

Without any other evidence to the contrary, that is very likely what has happened to yours. It's hard to know if your irrigation program makes sense without knowing your soil conditions or how long the irrigation runs for and at what volume. Once a week may be too little or too much....or maybe just right :-)

I'd suggest you dig up the dead one, look at the roots and examine the soil that was surrounding it. If the roots look off color or mushy, if the root ball has a musty odor and the surrounding soil is pretty damp, then it's a pretty safe bet a root rot was the cause.

@rhizo_1 Yes, these plants wilted first (similar to partially dead plant photograph).

In that case, Rejeev, I would take a look at gardengal's post. That's why I wanted the information about the wilting and dying in sections. So common to Pittosporum, sad to say. A couple of disease species are likely culprits.

this shows why planting a monoculture can be problematic ...

i would suggest you fill in the spots with anything but Ps ...

ken