Alice,
Please keep in mind that for some reason the connection/token from MS Flow into your ArcGIS Online account will be lost every 4-5 weeks and you will need to re-establish a connection; or at least that is what has been happening with me and few other users. I know that the Survey123 team has been trying to work with Microsoft to resolve the issue.
When/if it happens to you there will be a bunch of numbers and text within the module where the Survey name once appeared.
Once this happens, then you will need to do is open one of your Flows, tap on the Survey123 connector module, then the three dots, and add a connection. Sometimes it works on the first try and sometimes on the 5th or 6th try. You will have to click in the name where your survey once showed up to see if the list of available surveys shows up. Once this happens, then you will need to re-select your survey for the webhook to be associated to the survey again.
In my experience, when this happens you will need to open connect, re-download the survey form (this will help preserve the new webhook), republish the survey, and then have individuals refresh the survey in order for the webhook to work and function. After this occurs, I will then go into the "Connections" portion of the MS Flow website and delete the old and non-working connections to my AGO account.
I have not had this issue with Integromat.
Good luck,
Mike