Filter bags catch and hold lint. Unfortunately after a time they are not easy to clean. No matter how particular you are, the bag will not catch the lint as well as it did when the filter was new.
To test the ability of the bag to catch lint, firstly we need to understand how it works. While the machine is agitating, the surface water is drawn down through the centre of the agitator and passed out from the bottom. Because lint floats on the surface, it is drawn down through the filter and when the filter is clean the lint is captured in the bag.
When unbroken fats and undiluted fabric softeners remain in the filter bag, the water holds in the bag allowing the lint to float, in turn less water is drawn down through the bag and so more lint stays in the wash bowl.
The clothes being washed are moving around with their fabric softened and the pile is lifted. The floating lint passes through the loops of the fabric and will, in most cases, catch just as it should in the lint bag.
We now come back to how to test the lint bag. Hold the bag under a running tap and when new, the water will run straight through the bag. When the bag needs replacing it will hold some water and in some cases fill up.