Nea created documments are not deeing replicated

Good day to everybody, we have two servers of Couchbase with replication from A ->B. If we create documents from APP they are not being replicated and in the logs I see that “docs_failed_cr_source” is increased. Checked logs and don’t see any relevant error. If I create a document from UI the document is replicated, or if I update the document it is also replicated Could anybody advise what to check to fix this issue.
Couchbase 6.6.1-9213-1
OS Debian 10

I have the same problem, when I add new data to the bucket, some of them are not replicated after a while, if I update it again then sometimes it is replicated sometimes not. No stability. How can you detect the problem what kind of logs to watch?
Couchbase 6.6.1