Let's start at the end of treatment. Your braces are finally off and you have a new retainer to wear. Instructions are given for their wear and care and off you go. You come back 6 weeks later and your teeth are going crooked. Mum swears that you have been wearing them (because you told her you did) and she is not a happy camper. What happened? Let's be honest. You have not been wearing them exactly as your orthodontist told you. Either that or you lost them, the dog ate them or you melted them in the dryer when you left them in your pocket.
Face it, teeth do not move if you wear your retainers as instructed!! Have some self disipline and wear them. Save you, your parents and your orthodontist some grief by taking good care of the appliances that are going to keep your smile inatact.
Mum your child is not wearing their retainers if their teeth are getting crooked. If they say the retainers don't fit, it is because they didn't wear them adequately and the teeth moved. Believe me, the retainers fit on the day they got them or they would have complained then, also the Doctor doesn't let you leave unless they do fit perfectly.
So wear your retainers as you were told and everyone will be happy!