We have had issues with the forum wherein someone would comment and it has been saved, but after a while, the comment is gone. It was a long comment and it's disheartening to know that all efforts of the poster were gone. We did not touch the forum in any way, so I am not sure what is happening.
By "comment" you mean reply ?
If so, check your debug.log. If this is Enabled and Clean, then this seems like a caching problem. And i do remember you had some caching issues before.