1, short striker nose .
2, weak mainspring.
3, dried up grease or oil.
4,short sear so the hammer is not cocking to its full potential, a not uncommon fault on small bores.
5,too deep rims .
6, dragging or fouling of tumbler/hammer or cocking dog .
7, too much bounce or rebound on the tumbler , rare but not unknown .
These are the first things to have checked .After that I always blame the cartridges .