Check your class namespaces.
If class have namespace be sure you properly define them in comments or by alias.
Comments example:
...
/** @var \vendor\namespace\Class1 */
private $firstClass;
...
Alias example:
use \vendor\namespace\Class1;
...
/** @var Class1 */
private $firstClass;
...
Check project directories definition.
If your class directories not defined in project settings, PHPStorm can't index those classes. In this case code completion will not work. How to set up project directories in PHPStorm.
Clear PHPStorm cache.
Sometimes PHPStorm cache became invalid. Because of this, codecompletion may not work properly. Official how to cache cleanup.