Depending on the Transparent Peer Review configuration of the journal you reviewed for, there may be a number of reasons why your review, or the review page, is not visible on Publons.com.
The journal allows authors to opt out. If this is the case, then the author is offered the chance to opt out at each revision, with the final submission being definitive. If the author opts out then Publons cannot publish the review history for that paper.
You did not opt in at every revision. If you are given the option to opt out, then you must opt in at every stage in order for the review to be published. Failure to do so will result in the whole review history being unpublished.
Another reviewer did not opt in. If reviewers are given the option to opt out, then every reviewer must opt in at every stage in order for the review history to be published on Publons.com.
Rarely, an editor may also elect to withdraw an article from review history publication if any of the reviewer or author comments are found to be abusive, libellous, or otherwise inflammatory in a non-constructive fashion. Please note that this is entirely at the editor's discretion, and Publons does not screen comments prior to publication.