If your Magento instance is 2.1 or greater, there may be a system configuration which invalidates the access token after certain time period.
Directions are below to change the configuration:
To change validity of those tokens, you can follow below path:
Admin -> Stores -> Configurations -> Services -> OAuth ->
Select tab: "Access Token Expiration"
Will get 2 settings: customer Token LifeTime and admin Token LifeTime.
Set both its values to 0 ( token do not expired). You may have to uncheck "Use system value" checkbox to change its default values.