task failed without running

[heng gu]

I have this dag with a branchpythonoperator task kicking off many of 24 tasks, 
in this case, 4 tasks. 2 of the tasks were successful, the other two 
(register_YZ, register_ZY) were failed without running (see the attached UI 
screen shots). There is no log for tasks register_YZ and register_ZY. I am 
using Celery Executor and running 12 workers executing register_XX tasks. I am 
using airflow version 1.10.6. Any idea how to fix it?