Error validating access token: The user has not authorized application xxx

I am creating an app for users online shops where they can connect facebook integration for their shop by providing Pixel id, app id, app secret, access token. From our side we get page access token from it and save all the data in our db and then use it to send events. Everything worked fine for about 1 week but at 1 day 4 stores have a problem "Error validating access token: The user has not authorized application xxx" when trying to send event. I will appreciate your help in this problem.