In addition to Hassaan and Dhruv's comment I would like to make below comments:
1. Please make sure that volumes and ports belong to same resource group, i.e. if volume is from the Meta resource group then the port to which you are assigning this volume/lun must be in the meta resource group, please check it once.
2. If rebooting server also does not help to discover your lun then in the FC san switch try to disable and enable the port once.