when you omit priority
field in the JSON the default will be normal and that means firebase will send you the push whenever is appropriate.
and usually when your app is closed it takes more time to receive the message than when its foregrounded.
so if you are testing you can use "priority":"high"
to get things done quicker.
but remember to remove the field as it has high battery consumption for the end user
one more thing to consider is that on some devices (like Huawei) you have to make the app as trusted to use more power if this is the case you can do this under the power settings
Update
sending a push to a token sometimes wont work properly (since token change quickly during development time) and it's hard to verify token every time it refreshes.
So every time I'm testing the push I rather use a topic/package as "to"
parameter.
In case you subscribe your app to a topic you can check the working fellow using dashboard which suggests all the topics your app has subscribed to.