Firebase Cloud Messaging Bug. Sometimes not receiving any notification

I feel weird with this problem. I make a website with firebase notification. But when I try to send notification, 3/10 trial failed with success message (like: {"multicast_id":5735628470009762783,"success":1,"failure":0,"canonical_ids":0,"results":[{"message_id":"0:1487082201103751%cc9b4facf9fd7ecd"}]}) from firebase server.

Anyone know about this? and why?

My 10x trial: same payload, and every I send, it has success message from firebase server.

Thank you