spark on yarn,only one executor on one node works and the allocation is random

i run spark-shell with this command：

./bin/spark-shell --master yarn --num-executors 16       
--executor-memory 14G --executor-cores 8

i have four nodes，every node has 16G memory and 4cores

after i changed num-executors,the spark webUi tell me it worked, BUT only one executor on node which named "slave" is running

we could see TRANSFER1 and TRANSFER2 is empty

how can i solve this, when i submit job the situation has not changed should i change worker-instances or num-executors and so on