Certain sprint items not appearing in Kanban

[Jan Dan]

Hi guys.. We have had our workflow and sprint set up and functional for quite some time with no problems. However i discovered today that in some cases, a few of the tickets that are assigned to the sprint are not appearing in the kanban view. Theres tickets in every column, but for some reason, certain ones do not and I cannot fathom why, doesnt seem to have happened before. only common thing i can see on two of the tickets is they are attached to an epic, the same one. Anything there that could be causing problems? Or any other advice would be much appreciated.