I assume your talking about intellisence
Couple of reasons for this.
1: There is an error on the page that is not allowing the whole script to be analysed
2: The intellisence cache has not been updated since a change was made (ctrl+shift+r to force)
3: Third party intellisence app has expired
4: It's turned off. check the link
Configure IntelliSense (SQL Server Management Studio) | Microsoft Docs[
^]
Hope that helps :)