Why were some of the messengers messages lost when sending?

The most common reason for this is the loss of the messengers connection. When the API has processed the method and sent the request to messengers, a "watch" appears.
A clock next to the text of the message means that the message has not yet been sent from the phone and is waiting in line to be sent to the messengers server. If the icon does not disappear for a long time, this indicates a damaged network path and requires a check of the Internet connection. This leads to the loss of messages.