Security levels remind:
Performance - you can perform regular JQL search.
Strict - you can search for secure fields only by using "issue in search" functions.
Disabled - search is not possible.
Important: changing Security level always demands Jira re-index, otherwise changes will not be saved.
Common problems:
search that you perform
secure field's permission scheme
searching mode settings: Manage apps → Secure Fields → Search Security Settings
secure field settings: Issues → Custom fields → pick needed secure field and press Edit
configuration of the field: Issues → Custom fields → pick needed secure field and press Configure