Why does velcro strip lose its strength when cotton or fluff gets attatched to it?
Velcro works because it is made up of two different types of fibers, or fibers that are structured in two ways. The one side is actually made up of small hooks, usually stiffer and somewhat broader than the other side, which is actually made up of small loops, the side that looks hairier according to some.
If the hook side gets rubbed against cotton or other fluff and that fluff gets trapped because it often consists of something like the loop side of the velcro, the hooks are basically occupied by that other fiber or fabric and they cannot stick to the loop side of the velcro as well.
Velcro works because of thousands of small hooks and loops that become mechanically entangled when the two sides of the fabric are pressed together. If either the loops and/or the hooks get filled with some extraneous material, such as cotton fiber, they can no longer connect with each other and the effectiveness of the closure is reduced or lost.
The invention of velcro arose when a curious man wanted to know why burrs stuck to his clothes and his dog's hair. See link below.