Personally I do not think it's very clear exactly where you need to start your message.
There is a large clear box under the member's message to you, but it's not there you write, but further down and it's kind of greyed out a little, a slime box .... Send message, start your message there, then click on the blue forward arrow on the right when finished. I always tick the little box ' send message to my email.' this tells me it has actually been sent !
One more thing , if you are using the app or mobile browser, members are finding issues with those devices, use if you can a laptop / PC and Chrome that seems to work fine.
@monkeymagic71