iphone calendar not syncing with icloud

Hi

I am trying to get my iPhone 6s (running IoS 12.4) to synchronise calendar updates on Outlook 2019 with my Windows 10 Pro PC using iCloud. New calendar entries on my PC make their way OK to iCloud and then to my iPhone but updates on my iPhone calendar do not make it back either to iCloud or to my PC. Any ideas on what I am doing wrong?

Thanks

Peter