There could be different reasons which caused this behaviour.. How-ever make sure that you did cross-chek below requirements before assigning or when creating storage domain.
1. The LUN should be raw (no filesystem,no LVM, no data)
2. The LUN should be larger than 10Gb in size
3. The LUN should be r/w accessible from the SPM host.
Once you passed above checks, the Lun should be visible in the GUI. Also, if you have used that lun previously for storage domain creation, you will be mostly into “Point 1” in above requirement.