There are a number of reasons you may not be able to publish push notifications on channels.
Firstly, all channels that can accept push notifications must be explicitly enabled for push notifications with a channel rule. By default all channels have push notifications disabled. See how to set up and configure channel rules for more information.
Secondly, the publisher of the message on the channel must have permission to publish. See the authentication and capabilities documentation for more information.
Finally, if the push notification payload in the extras field of the message is invalid, it may be rejected immediately at the time of publishing. See the push notifications documentation for more info.