They might first "save as draft" and then (later) send. This is a standard feature, at least in KoboToolbox, and might be used until the problem is solved.
As LN asked in another posting:
Can you tell us more about the web form and how it has been distributed to the user? Is it a Public Access Link or is the user a Central Web User? Did you add /x/ to make it offlineable or is it a standard online-only form?
See here also, please: How should I distribute web form links to my data collectors? - #4 by LN.