It is also Limited in the amount of adjustment the orthodontist can make to increase the pressure but it is also the cheapest to make. It is possible if you have a clear plastic retainer it warms in the mouth to body temperature and has no rigidity to help hold the teeth an alignment. You did not mention which kind you have but I'll address all three. There are many different kinds fixed Invisalign and fixed plastic as well as bonded known as permanent retainers. Make sure that you're doing exactly as you were instructed and with that in mind let's talk about the kind of retainer that you have. This is also the time when control is in the patient's hand and not in the hands of the orthodontist so there is likely to be some difference in compliance. This is not true as retention is the most important portion of orthodontic success.
Many patients think that once they're out of braces the retainers are just an afterthought and not important. One of the most likely causes for relapse which is Shifting of the teeth back towards the original malocclusion would be non-compliance in the amount of time you're supposed to be wearing the retainer.
Depending on how difficult your case was whether or not it involved extractions for your braces your age or whether or not surgery was involved in moving your jaw structure depends on the correct reasoning for why your teeth may still be shifting. Your question begs for more information regarding the outcome of your orthodontic treatment.