I’ve come across a similar issue when testing contact us forms I’m building. The following might be what your users are experiencing:
During form testing, if I try and send two messages in a row using the form, it throws this error message:
The following errors were encountered
* You are not authorized to perform this action
Return to Previous Page
I can resolve this by simply refreshing the form and submitting again. It seems as though once the form has been cached, it can’t be used again until it has been refreshed.
I don’t know why this happens, but my guess is that it’s a built in mechanism to prevent using the form for spam. There are probably ways around this, like setting tag caching to “no” or something, but I haven’t played around with it so I’m not sure.