It's not uncommon for the paint to come off under high pressure washing. Did you use a hard epoxy based paint or ablative paint-which slowly washes off as the season goes on?
Like any other paint job it comes down to the prep work. I used to scrape, sand, and feather edge the bare spots before repainting. It still comes off. Now I just pressure wash the bottom in the spring before repainting. It's 10x easier and works out better for me.
I use a ablative paint made by Blue Water Marine Paint-it's slightly cheaper and works great.
Epoxy paint is supposed to last one season and ablative paint works year after year as long as you have a decent build up of paint on the hull. I give the bottom 1 coat every spring.