Navigation between Plugins

Summary

Plugin not loaded

Content

Hi ,
I am trying to navigate between plugins in Oracle field service cloud. by giving the folloowing close message sent bt plug in A to plugin B.
{
   "apiVersion": 1,
   "method": "close",
   "entity": "activity",
   "backScreen": "default",
   "backPluginLabel": "plugin_b"
}

when I was not adding the "backPluginLabel": "plugin_b" i.e when i was using the only pligin A it was working perfectly fine. but after adding this "backPluginLabel": "plugin_b" I am getting the error message that plugin has not loaded.
Can anyone help me out why my plugin is not getting loaded??