Why can I no longer access a book purchased from a Vendor ID when I already have my own Adobe ID? I can no longer re-download it either.

A few months ago I purchased this ebook: 180 Day Subscription: The Law and Business Administration in Canada, 14/e J.E. Smyth. I had to use a Vendor ID since I did not yet have an Adobe ID. Fast forward a few months later and I finally created an Adobe ID to purchase a different book. Only problem now is that I can no longer access this 180 day subscription that I previously was able to. Can someone please help me with this as soon as possible?