My Invisalign was working. But now my teeth are shifting back.

I'm on a tray 21/26. I have a crooked front tooth. My ortho made spaces in my teeth before starting Invisalign, and for the first few months I was seeing real improvements. BUT. Now I'm very worried. I feel like my tooth is moving back to its original position. The spaces all closed up, so maybe there's no where for it to go? It's also been VERY HARD to put on a new tray. It feels like the tray is too narrow for my teeth. Is that what's causing to front tooth to get crowded and crooked again?

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