Customers unable to submit form as it states "Waiting to send"

I am having numerous customers explaining that they have been unable to submit the form as it just states 'waiting to send' but does not send!

Hi,

I have sent you a test submission in order to inspect it in detail. Please ignore my test submission. I cannot replicate the issue. Please check my result:

I cleared caches of your forms. I would like to recommend you to refresh your browser's cache and try it again. How to refresh your browser's cache

If your problem persists, please let us know.

Thank you.

Kindest Regards.