Exterior painting of a home is almost never a do it yourself project.
The paint exposed to nature elements(sun, heat) and there is just too many reasons why paint could fail if not applied properly, or surface is not properly prepared.
House is not power washed prior to painting or the power washer doesn't have high enough pressure.
Usually power washer with less than 2000 psi(pressure per square inch) will not do a good job in power washing the house. Most gas power washers qualify. Most electric ones have less than 1500psi.
Not scraping loose paint by hand.
If the house has a lot of pealing paint, most of it will come off after the house is power washed. But power washing along will not solve the problem. Scraping loose paint by hand will ensure that all the paint that is about to fall off, will come off before the new paint is applied. Otherwise new paint will fall off with the chunks of an old one.
Not fixing the cracks.
Moisture will get inside the cracks and paint will start pealing.
Not spot priming repaired areas.
The paint will not come off if the surface is not primed. It will look fine and may even hold long enough. But most of the paints manufacured to be the top coat, and with primer underneath it will last much longer.
Not using the right type or of paint or primer. Or the right type but of a low quality products.
There is a variety of different types of paint for a reason. Rod iron prefers oil base paint, wood-100% acrylic paint. Not using the proper type of paint manufactured for a particular surface will reduce the life of a painting job significantly.

ABN Painting Exterior and Interior House Painting. 602-214-3040
Prompt, hardworking and reliable painters.