One of the most common reasons members lose posts they thought they had completed is caused by the member "moving on" much too soon after submission by clicking a link or using the browser's Back button, etc. It takes a few seconds, or more depending upon your internet connection and server load, for the web server to fully receive your submission as the content must be uploaded to the server. If you, your browser, your internet connection, etc. do anything that aborts or otherwise interrupts that transfer before it can complete then the server will detect the aborted transaction and you will lose your post -- from the server's perspective that is the best it can do for an aborted transaction since it was an incomplete submission.
On mobile devices, simply switching apps or returning to the Home screen can be enough to cause a mobile browser to abort the submission if it did not complete prior to switching screens.
So how do you know when it is safe to move on? Simply wait until you see the new screen following submission showing the thread you just posted to, including your newly posted reply to that thread thread. You may have to scroll up or down a bit before you see your reply if the new page fails to automatically scroll to your new post.