All primers leak eventually. In a new case with a smooth primer pocket, the first primer will seal pretty well, but the first time the case is reloaded, the old primer will leave small scratches in the primer pocket wall as it is pushed out. Primer pocket cleaning tools will often leave more damage. The next primer will seal less well and the situation gets worse over time. Pitting of the bolt face is unavoidable, but usually doesn't cause a problem untill the firing pin hole starts getting involved. Then the bolt face can be repaired by inserting a bushing with the correct firing pin hole size.
RAN
