Could Invisalign damage teeth and gums? (photo)

I started invisalign a yr ago. every dentist said I needed 4 teeth pulled. This dentist said they did not need to be pulled. in may of this year my teeth are loose so he had me stay in the same tray for 2 months. every time I put try in they fill with blood. he is having me go from tray 18 back to 12. I don't know how this is going to make teeth move that have no where to go. He keeps saying the teeth do not need to be pulled. they are going to fall out on their own, but not the right teeth.

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