At times, pull requests could sit idle for days, maybe weeks. Sometimes it'll be ignored when in reality, the branch needs to be merged. You could write a comment which will notify all reviewers but now your pull request has meaningless comments.
Create a Nudge button for pull requests. This will simply notify all reviewers that there's a pull request left unattended. Some info on pull request would be useful also:
Pull request date
Who has reviewed it (if the person has reviewed it, there will be a message in the alert that says "You have approved this but still waiting for others: John1, John2, John3).
A color indicator/flag that correlates to the length of time it's been idle: Yellow (3 days), Red (4+ days), etc.