Why is paint job on gables bubbling and chipping in large, plastic-like pieces after less than a year?

While it could have been a lack of primer, or poor primer, your description of the size and thickness of the pieces coming off suggests a top coat that stuck to primer pretty well, but moisture in the board caused the basic bond to fail. Such moisture is the leading cause of exterior paint failure.

I'd closely examine the roof and roof edge in the vicinity, particularly the drip edge. Water getting in there will wick through trim boards and kick the paint off. Another all-too common problem is using a power washer to do a quick "prep" for painting, forcing water into where it shouldn't go, and/or not letting the area thoroughly dry before painting. Quick prep = quick fail.

One way or another, have to get up there and get close to it, not just trying to reach up and slap more paint on. I'd plan to scrape off pretty much the entire run of trim - probably finding that a lot more is ready to fail, too. After addressing moisture issues, and letting the area thoroughly dry for some days, I'd use an oil-base exterior primer, or perhaps a "hybrid" like Zinssers' 123 (really versatile stuff). Then two coats of a good exterior water-base paint. Ought to last around ten years.

What kind of warranty was extended to you? If there is no WRITTEN warranty, all bets are off. If there IS a WRITTEN warranty (verbal agreements are unenforcable) then read the terms.

This is the problem with shopping for price rather than quality which is what I imagine occurred. Save your health, hire a respected painter, and pay the asking price. Again.

Is it down to bare wood? I saw a mansion that had this happen twice. They didn't use primer because new paints supposedly don't need primer which is a lie as far as I'm concerned. Cheap damn paint that isn't cheap. Like certain paints that get great reviews, probably from the manufacturer and are junk. Insist on a good brand of paint like Benjamin Moore not some 5 gallon bucket of generic crap.

There are a couple possibilities, #1- It wasn't prepped right and the chipped / bubbled spots should have been stripped, #2 - The previous paint was a different type, (For example putting on a water based paint over an oil based paint).