Retention. Instability may be due to severity of the original malocclusion, partial instead of full retainer compliance, poor oral hygiene caused damage to supporting tissues, and expansion of teeth and alveolar bone beyond basal bone support. Assuming Rx provided by qualified Orthodontic Specialist return for evaluation/adjustment ASAP. Delay makes it worse. If treated by a GP, see an Orthodontist.
Improper placement. A retainer that is not inserted fully into place will allow the teeth to shift slightly. Once that happens, it never really fits right, and the teeth continue to shift. Although the patient is "wearing" the retainer, it is not doing it's job. See your orthodontist regarding options to correct the situation.