Why does a gap reopen even with permanent retainers on?

Check retainer: Depends what kind of retainer. With a bonded lingual retainer there should be no gap. If the retainer is removable then it is not fitting correctly and needs to be refit or remade.

Frenum: If you had a gap between your front teeth it is possible you had extra gum tissue in between them. This tissue is called a frenum and can act like a piece of rubber wedging the teeth apart. A frenectomy , preferably with a laser may be indicated.

Gaps : Permanent retainers only hold certain teeth together. The others can still move. It also depends upon how close to ideal your bite was when your treatment was completed. See your orthodontist to see if the permanent retainers have debonded or broken. You can also then find out specifically why you developed gaps and what can be done to close them if that is what you want.

Loose: A "permanent" retainer is attached to the teeth. If the teeth to which the retainer is attached are moving, then the retainer has become detached. Please immediately see the Orthodontic Specialist or General Dentist who treated you.

Forces: Teeth are dynamic. Unless a person is dead and no longer functions, when you eat, chew, brux, grind, clench, all of that are extreme forces and teeth are responsive to those forces. If you have a bad bite or malocclusion, teeth tend to drift as well, or tongue thrusting, or even new restorations, fillings, etc. may cause shifting of teeth.

Ill Fittin Retainer: My recommendation is to see your orthodontist as soon as possible for evaluation and get a new retainer made that fits properly.