Why are user contributions on the profile page empty?
The plugin uses email to match Bitbucket user and commit author. So, there will be no entries if the email associated with Bitbucket user doesn't match the email of the commit author.
Bitbucket doesn't show emails of commit authors in UI so it may be required to clone the repository and use git commands like git log or git config user.email to show user email.
For example, clone the repository locally and then use git show command:
Possibly the user configured the wrong email address in his local git setup, correcting this address will solve the issue for future commits.
The user contribution page will show the entries of future commits after the corrected address is configured.