A few common issues that will affect Intellisense:
- Use a single root project. In other words, open the directory that contains the top level of one project (where the project.properties file is), not a higher level.
- Make sure you have a fully configured Particle project, containing the project.properties file and the .vscode directory with settings. The easiest way is to use Particle: Create New Project.
- There are a couple more tips in this post that may or may not help.