In the first 6 months after the end of active orthodontic treatment, the retainer feels tight every time you put it in, which means the teeth have moved slightly while you were not wearing it. This is because the bone is still soft and allows it. After this period, you shouldn’t see any visible shifting of the teeth during the day when you are not wearing the retainer. If this still happens, it is most often due to strong cheek muscles pressing on the dental arches and trying to narrow them slightly, in which case it may take longer for the teeth to settle. Another possible cause is the contacts between the teeth, which push them slightly out of line. Lightly filing certain contact points can solve the problem.
This answer is for general information. Your own case needs an examination by an orthodontist.