Q: Birthdays in Contacts: some appear in Cal, some do not

I have subscribed to Birthdays in Cal and have entered many birthdays into my Contacts.  Some of these appear as expected in my Calendar, but others do not.  I have delete and re-entered them, but this exercise makes no difference. "Birthday" is no a default entry in Contacts, but I have checked my spelling in each case.   I am also confused about whether there should be separate entries for contacts "On my Mac" and on iCloud.  -jw