Problem forwarding from iCloud to Gmail

I don’t really use iCloud mail, but I have iCloud email addresses that I often give to people. I have all of my iCloud mail forwarded to my gmail. Recently, I have noticed that some messages are not being forwarded. At first, I misdiagnosed the problem as an overstuffed iCloud mailbox, based on experience I had a couple of years ago. But, I now think that was a red herring. Some messages are getting forwarded and some are not. I now suspect it has something to do with DMARC. Is anyone else having this problem, and do you have any ideas on how to solve it?