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It will be hard to know without photos of the hedge, and closeups as well. Sometimes, they just need a hard pruning to bring it back. You may need to fertilize, but this is rare.

Usually, regenerative pruning will be your best option. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/pittosporum/growing-japanese-pittosporum.htm

Dig a trench around the shrub or tree using a sharp spade to the depth needed for the root ball. For example, a 4-foot-tall shrub needs a 16-inch-deep root ball.
Move the pittosporum to its new location.
Dig a hole twice as wide as the root ball of the pittosporum you're moving and deep enough to allow the soil line evident on the pittosporum's trunk to sit an inch above the ground.
Place the pittosporum in the hole. Fill the hole in with the removed subsoil. Water the transplant to settle the subsoil.
Add topsoil to fill the top 6 inches of the hole and tamp down lightly. Mound additional soil in a circle around the plant to hold in moisture.
Check the pittosporum daily for the first two to three weeks to see if it needs watering. Consistent moisture levels ensure that the pittosporum grows well in its new location and regular checks ensure it isn't over- or under watered.

https://www.gardeningknowhow.com/ornamental/shrubs/pittosporum/growing-japanese-pittosporum.htm

You can treat for a fungal infection.
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

In Pittosporum, foamy canker is usually an alchoholic flux that is a result of water deficit/drought stress or other environmental stress factors. It is usually not a pathogenic disease condition and is not a killer by itself or reason to remove the tree. Although the stress factors that are creating susceptibility to the alcoholic flux could be serious enough to cause decline and mortality.

"Providing plants with proper cultural care and preventing injuries to bark may help to prevent foamy canker."

http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/foamycanker.html

https://www.gardeningknowhow.com/ornamental/shrubs/pittosporum/growing-japanese-pittosporum.htm

Hm. This looks to resemble a soil infection. It is hard to control the weather, but they do not like it to be overly wet.

I would recommend testing the pH of the soil, then adjusting down to about 5.5 to 6. They prefer a rich acidic soil, so adding compost would be better than nutrients, since most nutrients are designed to be neutral in pH. Pittosporum will not tolerate this near as well, but will still grow at a higher pH.

You can adjust the soil pH down with Iron Sulfate, which will also kill the infection off.

This article will help you to care for Pittosporum: https://www.gardeningknowhow.com/ornamental/shrubs/pittosporum/growing-japanese-pittosporum.htm

Here is an article that will help you with fungicide use, which will help when sprayed on the upper portions of the plant: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

You don't say what species of Pittosporum you have, but in coastal California, Victorian Box have been dying from a syndrome called "Victorian box decline." The following linked article will give you an idea of the multiple factors that may be contributing. These cultural factors are important for plant health no matter what the species, so I would recommend that you address good soil fertility and water management first, in order to relieve the stress factors that may be making the trees susceptible to decline.

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=16512