When you create a bulk inscription, you are essentially creating several individual inscriptions at one time, which do not belong to a collection.
When you create a Public Mint collection, you cannot add to the collection over time. The creator doesn't cover fees up-front, and collectors pay to inscribe the ordinal when they mint.
Adding single inscriptions to a collection
In the screenshot below, you can see this inscription isn't part of a collection.
If you would like to request a collection page, follow the instructions in this article.