I have four callistemons which I planted 25 years ago. Tradionally they have flowered well but over the last few years we have had minimal flowers. Should I replace them?
You could replace them; 25 years is a long time. If you want to give them one last shot, this article should help:
https://www.gardeningknowhow.com/ornamental/shrubs/bottlebrush/bottlebrush-wont-bloom.htm