Not receiving forms from tablet/phone users

Twice in the past month I have not received a contact form that customer has filled it out. In both cases they received confirmation that we received their form. We asked both times, for the customer to try filling out the form again, and we still did not receive it, thought they again received confirmation.

They called us about it, but we may not have heard about it if they hadn’t. So, there may be others.

One of the customers used an ipad and the other used their phone.

I am using wordpress 4.0.37

https://wordpress.org/plugins/si-contact-form/