Form submitted, but not received

I have a large number of people submitting the same form. For 99% of the people it works fine. I receive the form, and they receive a confirmation, just as I designed it in Gravity Forms.

However, there is still that 1% who keep trying to submit the same form as everyone else but it doesn't work. They press the submit button but I don't receive the form and they don't receive the confirmation email.

What are some possible causes for Gravity Forms to not work for some users?

What kind of questions can I ask them to help troubleshoot the issue?

Does this happen to anyone else here?

I continue to have a small number of users who tell me they submitted the form, but I did not receive it. I suspect that it has to do with conditional logic because of the many forms I am using, the "problem" form is the only one that has conditional logic in it. I have disabled the conditional logic on the form and I'm running a test with few of my users. I'll report back with the results.