Incomplete list of folders and files with Google Drive REST api v2

I am working on a Python app which gets access to Google drive. I use the official api.

Here's my code for retrieving list of root folders and files:

files = service.files().list(q = "'root' in parents and trashed=false",
                                         orderBy = "modifiedDate").execute()

The strange think is that I get only some of the folders. I just can't figure why the rest of folders are not present in the result set. I am using v2, and did not test whether the same issue is present in v3. Any ideas why this could happen ?