Hi! I have evaluated that this could be caused by not very good internet connection.
However, it creates another quite important problem. There are a lot of places, especially outside a city, where the internet connection is not very good. And now it is hard to understand for app users whether their form was submitted or not. The only approval of submission is this small appearing window with tick mark, which sometimes do not appear at all.
I think, that it is necessary to make another indicator for the user that the form was not submitted and, ideally, “Try again” button. It is critically important for an app that takes orders because app owner can have problems with orders that have been sent by the client but not received by the google sheet.