Is it possible that your data for that field doesn't have more than 5 distinct values? The "Other" bucket will not show if the count of other values is 0.
You can check the distinct values of a field by running a query like this in Console, where logstash-* is your index pattern, and machine.os.keyword is your field:
GET logstash-*/_search
{
"size": "0",
"aggs": {
"uniq_values": {
"terms": {
"field": "machine.os.keyword"
}
}
}
}