I’ve finished Invisalign treatment and have been wearing retainers every night for over a year. After about 3 months my retainers changed shape slightly, which changed the position of my teeth. I’ve gone through all 3 of my retainers and each one has lost the original positioning of my teeth. Is this normal? And how often should retainers be replaced? I feel like I need another course of Invisalign treatment to refine my teeth again.