As a guest editor of an international journal, I was supposed to handle the review process of submitted articles. Two of the reviewers do not respond even after sending couple of reminders. I had clearly mentioned about their availability and timely review process at the beginning. I aware that the reviewers are often very busy with their academic overload. Thus, it is very common about the delay. However, from editor's point of view, it has to be make sure about timely review process. So, it is ethically right to ignore the reviewers and search/assign new set of reviewers?
I have also found that some reviewers do not respond after the 2nd time submissions for a paper suggested for resubmission. Thus I have faced the similar issues with different prospect.