somewhat difficult to speculate on things with the lock out of the gun. the cock most likely has traveled beyond it's normal range without a nipple present to stop it allowing the main spring to appear that it would make contact with the bottom of the mortice. have close look at the mortice floor and see if it looks like the spring is making contact. there should be some marking an burnished looking wood if the spring is banging into the mortice floor. quick/easy and perfectly acceptable remedy is as suggested,....a longer nipple. the one in there now, might very well be a replacement that is slightly too short. it can sometimes be proven out by the angle between the cock and the nipple, given that proper geometry should have the cock striking the nipple straight on. a longer or shorter nipple will allow that alignment to change. another method might be to snap a cap and look at the shape of the cap's top. if severely wedge shaped, the nipple is not the correct length. the gun will most likely fire fine this way,.... but,...it is not correct. there is also the possibility that the cock has been replaced at some time, with one that has an incorrect nose length, or s slightly off in clock timing of the hole. essentially,...with the gun's age,.. there is no telling what might have been changed at some time. all you can do is spot the problems and fix them.