Randomly Sending Form Submissions

I have 2 forms installed on my site.
It is a brand new site with no other plugins currently installed.
When I test the forms, sometimes they process — take me to the Success message and generate an email which is sent & received.
But then if I try to send another form — either the same form or the other form — nothing happens. Success message does not display, email is not sent/received.

I have tested this on multiple computers, multiple browsers, with the same results. Usually, the first form submitted on that machine/browser processes, all consecutive attempts don’t.

Most recent attempt, though, the 2nd form submitted went through. But the first submission did not.

I am stumped.

(Evaluating the plugin for possible purchase and getting discouraged…)