CF7 does not send forms in 50% cases, gives an error “FAILED TO SEND YOUR MESSAG

My CF7 is sending forms in 50% and in other 50% it does not, it gives a red error message, usually in a red box: “FAILED TO SEND YOUR MESSAGE. PLEASE TRY LATER OR CONTACT THE ADMINISTRATOR BY ANOTHER METHOD.”
I also have a fully set up WP Mail SMTP plugin. My emails are with G-Suite. I have all plugins updated. I contacted my hosting, they don’t see any problems at their end and sent me here.