Getting same issues for my client. Someties getting mail sometimes not. Forced to switch to CF7 (Contact Form 7) and make my own multistep form that works. Just to let you know getting same issues and when I use CF7 works without any problems. Guys I assuming that you have some issues somewhere with caching system with AJAX. Maybe. I just asuming and not 100%. Didn’t checked in taht details.
Tried to disable all plugins and same issue. The annoying part that works totally randomly. Sometimes get mail message sometimes not. With plugins enabled and then all disabled. Really tried to locate the issue but this random working not giving any logic to try to locate issue. That why I wrtting here.
Might I can give some clue to you. Here list of used plugins:
Cloudflare
Google Analytics for WordPress by MonsterInsights
Jetpack
Limit Attempts by BestWebSoft
Redirection By John Godley
SG Optimizer
Yoast SEO
Hosting Siteground with their server cache enabled. Only thing that I can’t disable their cache system. If I dissable theri cache system it will get warning and site will be suspended due theri server resource restriction and high trafic that have. Already had problems with that.
I switched to CF7 and made my own version. I thoought I will save some time with this plugin but the crucial part is not working. By the way it is great plugin. In short time setup and style it to fit to the site. Only the mail delivery problem forced to end using this plugin.
If any additional info is needed that you think might help to locate the sissue let me know. I gave a chance to this plugin but failed in most basic part.
It would be great to locate this and fix it. At least for future project can re use it when is delivery time is short.