There are various limits set on the BONUS+ central server to prevent holds being placed on items that are not likely to be available soon.
There are also limits on the numbers of holds that can be placed by a client:
If the patron's home library also limits the number of holds that can be placed by their clients on their collection, any active BONUS+ holds are counted towards that total.
The owning library hasn't coded the multi-volume work so it can be specifically requestable. For individual volumes to be requestable, multi-volume works need to be coded with volume information in the item record volume field attached to the bibliographic record,