Why does the film peel off?
Clean the surface thoroughly and make sure the surface is clean enough without any dust.
Make sure to remove the first tier of the protection film before applying on the window.
How can the film stick well and lets bubbles disappear?
Use enough water. The more flowing water, the better. Squeeze out excess water and bubbles with scraper or hard card from middle to edge.
Is this window film reusable? Will there be any glue marks left after tearing it off?
Yes, reusable. No adhesive, water, scraper, and knife are the only tools you will use. Easy to apply and remove without any residue left on the glass.