delete operations don't sync from mongo to elastic

[hao-opentown]

I've set up the cluster successfully, the insert and update ops in mongo will be reflected immediately in the next elastic query. But the delete ops in mongo never appears in elastic, i.e. elastic doesn't delete anything.

I also check the transporter's transformer log, which shows the deleted ops from mongo is logged well and ready for elastic to use.

transformer: {"data":{"_id":{"$oid":"5617763fe2b97de59d269ee8"},"content":"haozi.","title":"Breaking news"},"ns":"harvester-test.entries","op":"insert","ts":1444378175}
transformer: {"data":{"_id":{"$oid":"5617763fe2b97de59d269ee8"},"content":"lucas","title":"Breaking news"},"ns":"harvester-test.entries","op":"update","ts":1444378211}
transformer: {"data":{"_id":{"$oid":"5617763fe2b97de59d269ee8"}},"ns":"harvester-test.entries","op":"delete","ts":1444378780}

Why does it happens?