Sometimes I cannot send the message on any browser based on Chrome (Chrome 58-59, Yandex 17, Vivaldi 1.9). It has happened mostly in the past couple of months. It works 100% on Firefox and Edge. I am not sure whether it's the problem of Chrome or the site. According to my usage/observation, only MDL has issues (exception - Vivaldi, many sites are not displayed correctly).

Are you clicking it twice? I have noticed that when I hit send, nothing happens. Then once more and it is goes through. Kind of like Chrome is asking if you're sure you want to send. :p It doesn't always happen, but more often than not it for me. Unfortunately, most times when it doesn't happen, I end up double posting. >_
I click 10-20 times and still nothing happens. Earlier if nothing happened and clicked twice, it used to go through and sometimes twice or more! Also, this happens only with the browsers related to Chrome. Right now, I'm using Edge and just sent a message with single click. Before that I tried Chrome and hit the button 4-5 times and nothing happened.