Just repainted my old home (1923) – lap siding – used Behr paint w/primer included — scraped off all old paint. Now bubbles and blisters have formed. What caused that and how can I prevent it from happening more and more?

Someone I know removed wallpaper and did not remove the adhesive. They primed and painted over it. Now … I’m trying to remove it all, and used a paint remover to start. Well, after scraping for several days, the surface is visible in some places, but the whole thing is