The hotkeys specific to smplayer (like next video etc) doesn't work after you switched to a new video.
So if you are watching video number one in fullscreen, then use the hotkey 'n' to go to the next one, that hotkey will no longer work. If you however, move the mouse, so that you trigger the floating bar, that re-enables the hotkeys once more. It seems to me that the next video call, should directly enable the hotkeys, or possibly just trigger the floating bar.