There is “Message Requests” folder on Facebook, in which messages from people you do not know (Those people who are not in your friends list) are stored.
Messages from people you do not know are kept in “Message Requests” folder, you get notification as soon as the message arrives there but sometimes, messages arrives as spam and you do not receive this notification every time. So your important messages may be laying there.
go to your messenger app, and reach the setting option, tap on it and a drop down box will appear. You will see ‘people’ option, after going on that tap messenger request. There is your ‘Other’ folder and the ‘filtered messages’.
So to solve this, go to your Facebook messenger app, settings option and then tap on it, a drop down box will appear where you will see “people” option, after going there, tap on messenger request, there is your “Other” and “Filtered messages” folder.