I diassembled the lock and... I understood
the problem is not about the pick!
I chose a difficult lock when I bought it. The 5th pin should be deep set to reach the sheer-line while the 4th pin should be high set.
But the actual problem is:
The 5th pin is far longer than others, since it is deep set. Thats why there is a very little space between the warding and the 5th pin. As a result I can't make my full hook enter this distance without applying too much pressure!
On the other hand, from what I learned, I know that I need a full hook to set that deep set pin behind high set pins...
I'm really confused!