Well, I think I just had bad luck the first few attempts trying to pull it on the shore. 3 of the encounters happened right before the dawn, when the sun raised they immediately turned away and tried to get back into the ocean. Another time I was not prepared, run out of stamina on the shore and let it slip away. I managed to kill 3 serpent afterwards with the help of stamina potions. Also the spawn location is important, if it's in the middle of the ocean with no land around, it's just impossible to harvest its scale.