Hey Alberto, you are not alone. It has happened to me. I don't know what causes it but I sometimes do that when I am between patterns or I have tyed most of the patterns I have on a list I keep. I found a few things to break it. One is to start cleaning or organizing your fly tying area. I usually come across some materials I bought that I forgot about and decide to tye some patterns using the material. It also helps to check the archives here for a pattern that catches your eye. That gives your a pattern to focus on at the bench.

Hang in there!! I with ya!

Dr Bob