Someone answers a typeform form but it doesn't appear in typeform responses

Hi, wondering if someone has had this experience or a possible explanation? Many thanks!

We have a Typeform questionnaire, and when people submit  it, we web hook the answers on our back end.  So the answers appear on the Typeform results tab, as well as within our WordPress backend

This morning we had someone complete the Typeform (presumably), as we saw it on our WordPress web-hooked backend, including his name and email and some other info entered. 

But...and this is the odd bit! We could not find his form submission in Typeform results/answers.  

Has this happened to you?  

Would love to understand it.  

Cheers,

Bela