Multiple items in same year by same author not listed correctly


I’ve just updated the citations and bibliography and I’ve noticed that where I have multiple works by the same author in the same year, instead of listing as (2004a) (2004b) they’re now listed in an odd order, both in the in-text references and in the bibliography. In one case it lists the references as  (2004c) (2004f)  and in another (2000a) (2000m). I’m not aware that I’ve made any major changes to the EndNote library other than tweaking a few references and adding some others

Any ideas how to resolve this?