Paging mismatch between the client instance and the LDAP server could result on missing records when large queries are performed. The LDAP server can limit queries on the server-side, but for LDAP Server with versions 3, you can use pagination. Pagination allows you to make repeated smaller queries that overcome this server-side limits and retrieve much larger result sets. A large import set of users from LDAP servers that do not support paging can partially fail.

• Setup an LDAP directory that do not support paging.
• Create 5000 users with sAMAccountName with 8 digits starting 'ZZ'
• Configure this LDAP server on ServiceNow
• Configure the OU to have a filter of (sAMAccountName=ZZ*)
• Load all records in data source

The result is that only 1000 users are imported instead of the 5000 users available.

The solution is to activate your LDAP server support for LDAP v3 pagination (rfc2696).
Alternatively, split the LDAP OU Definition on smaller queries (less than 1000 records per query).