Just a simple thought: Could it be something like HISTIGNORE or 'ignorespace' in HISTCONTROL? After I had one too many lines disappear to "ignorespace", I decided it wasn't too worth it.
If I type history at this point, I'll see most of my command history ... with an empty line in place of the line I was hoping to repeat. The command is gone. My only option is to remember what I typed, and type it all again."
