It is possible that she has snuck a shed in without you noticing. Sometimes snakes shed and the shed is hidden in the layers of the substrate if using newspaper, or something similar. You could have changed the paper with a shed hidden inside and not realised. It has happened to me on a few occasions.
Otherwise, she could be not eating enough, and not grown significantly, or be too dry and be needing more humidity.
The are a number of things that could be the issue, maybe post some pics of her and then it may be easier to tell if she is overdue for a shed, or if she is OK and maybe the shed was missed, or if she has not grown enough and is still comfortable in her current skin. As said, older animals do shed a lot less frequently than young ones that are constantly growing.