fast retransmit does not work

Fast retransmit says that if 3 dupacks have been received by sender, then the sender should retransmit the lost packet. But my tcpdump output shows that it didn't work like this. It didn't retransmit the packet even more than 10 dupacks have been received. Why didn't it triggerfast retransmit? Does anyone has idea about this? Thanks `cat /proc/version: Linux version 4.2.0-42-generic (buildd@lgw01-55) (gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.3) ) #49~14.04.1-Ubuntu SMP Wed Jun 29 20:22:11 UTC 2016 ` `sysctl: net.ipv4.tcp_congestion_control = cubic`