Problems occurring on only some browsers or devices is relatively common with JavaScript Conflicts.
Step by step approach to finding JavaScript Conflicts
1. Switch temporarily to a WordPress default theme (Twenty Eleven etc.) and see if the problem goes away.
If it does, you’ve most likely got a JavaScript conflict with your current WordPress theme.
If switching to default theme doesn’t help it could be one of the plugins you are using that is causing the problem.
2. Disable all plugins other than Contact Form 7
If doing that solves the problem, you can then add back each item – one by one, until you find the real cause of the problem.