Not able to search text partially with provideCompletionItems

I am working on an intellisense extension in VS Code. My use case is to show SCSS variables that are already available in one particular file (like variables.scss).

Let's say I have a completion Item defined like this

vscode.languages.registerCompletionItemProvider('plaintext', {
       provideCompletionItems(document, position, token) {
           var a = new vscode.CompletionItem("$white: #fff;");
           a.insertText = '$white;';
           return [a];
       }
});

If I type f in a textfile. I am not seeing any list of intenseness. But If I type w I can see $white: #fff; Again If I type wf, I can see the following highlights:

with f typed-in: with <code>f</code> typedin

with wf typed-in: with <code>wf</code> typedin

I need to see a list even if I type f but I am not seeing any. Any help is appreciated.