The Guppyfriend Washing Bag was designed to reduce microfiber loss at the source. The structure of the filter surface is optimized to achieve the smoothest surface possible, to protect your garment, to avoid pilling and to reduce fiber loss. (see also: Who tested the bag and what are the results? )
Mechanical forces, one of the main causes of fibers breaking, are reduced by the softness of the filter mesh. The microfibers are extremely tiny and barely visible to the naked eye, and most of the (added) plastic fibers in your textiles are transparent. So, it is hard to detect them, especially in a wet filter bag.
Hence it is perfectly fine when you don’t find microfibers at first. But they will collect over time. You’ll find them mostly in the upper corners or the seam.
But don’t expect to find a lot. It’s the few tiny microfibers that make their way from each washing machine into rivers and oceans that – taken all together – cause harm to oceans and sea life.