This problem seems quite normal. Over time, the little holes are filled up with small coffee grounds and stuck there by the help of glueing force of grease. This grease is extracted during brewing process.
So, how could we get rid of these remains?
Pretty easy. As they are organic compounds, you should dissolve them. As they include grease, dissolve the oil that includes particles.
Best options are:
- Acetic acid, thus vinegar
- Citric acid, thus lemon juice
These are food, so safe to use. You may leave the filter in these for a while. Apart from that, you may try alcohol, but check whether it damages the filter.
Last option may be bleach. It dissolves any organic compound quite easily as it is corrosive. However, it is not safe. You must rinse it thoroughly.