Jeff Strunk wrote:

One example is adding the olcSyncrepl attribute to olcDatabase={1}hdb,cn=config . Ldapmodify reported success. Ldapsearch shows the new attribute. syncrepl works. However, /etc/ldap/slapd.d/cn=config/olcDatabase={1}hdb,cn=config.ldif does not have an olcSyncrepl attribute. When slapd is restarted, the olcSyncrepl attribute is missing.

The same thing happened when adding the entry for the syncrepl overlay.

Have you successfully run "make test"? Those exact operations are part of test050... Have you got any slapd debug messages from these modification attempts?