Every time Candace tries to bust her brothers, her mom always can't see the invention or structure.

This is because Phineas is dead. He got bullied so much in school. When one of the bullies punched him so hard, he died. Candace got scarred and hurt. So, she created a world when, he's still alive and building structures with his brother. When Candace tried busting them, mom can't see anything because it's not real. Her mom got so worried that she brought Candace to a psychiatrist who is Dr. Doofenshmertz (hope it's correct spelling). She visit him two times a week and he keeps on checking if she's alright.

Just. Wow. It can explain it.. But, the main cause of disappearing structures is usually Perry. But, maybe... Perry doesn't exist in this theory... Hmm...

9/10