Invisalign liners feel loose after 2 weeks?

I have recently begun my invisalign treatment, and have noticed that after two week my invisalign liners feel loose and almost worn out. The issue here is that my orthodontist instructed me to change aligners every three weeks. I am aware that most people keep their aligners in for only two weeks, which seems to make more sense to me. I am fourteen with slight crowding and only ten aligners for both my top and bottom teeth.