Hi Laura,
Has the document been released? The document can be released with a defined release interval. Based on what I understand, the document simply expires after this end date without a new version, right?
If the document is released and release interval shows "not released", then try adjusting the "release attribute" in customizing setting below:
SPRO > IMG > CA comp. > DMS > Control Data > Define Document Types > Select the Doc Type in question and go to Define Document Status.
Regards
Anil