Calendar on Macbook Pro and iPhone 11

I have a Macbook Pro, 2019, running OS Big Sur, 8 GHz, 8 GB memory and an iPhone 11. I have Calendar installed on both. If I create a recurring entry on Calendar in Macbook Pro, I cannot alter this entry (other than to delete it) - the Pro will not save the changes. Yet, when I look at that same entry on Calendar on iPhone, it shows that it has been altered. The only way that I can get the entry changed on Macbook Pro is to change it on Calendar in iPhone. Is there a way to fix this? Thanks in advance.