AFAIK the default OpenLDAP response limit is 500 entries. The client (UI) supports paged results. Perhaps the server does not.
Do you have any error in /var/log/messages?
BTW, assuming you have a local OpenLDAP accounts provider, you can increase the response limit with
ldapmodify -Y EXTERNAL <<EOF
dn: cn=config
changetype: modify
replace: olcSizeLimit
olcSizeLimit: 800
EOF