XDCR seems to be working but there is difference between item counts

I’m have 2 couchbase clusters (3 nodes each). The nodes are running on AWS EC2 instances.
I set up bi-directional XDCR replication between clusters.
Yesterday something happened and despite I see that there are metrics that XDCR is working and something is syncing I see gap between the item count in the bucket on the first cluster and the count on the second one.

I have Community Edition v5.0.1.
What can be the problem and how can I fix the gap.