I posted a pic of mine recently, I run the top of the water just below the top of the main skimmer body. It's a combination of not having it run too high and skim too wet as warby says, and having enough water in the cone for good contact time. The bubbles work better with a longer contact time, hence tall skimmers work better. I've seen the auto scrapers, they look to me to run the risk of knocking the crud back down into the water, though I've not seen one working in person and I could easily have it wrong.
Do you clean your cup in RO or tapwater? I saw something a long time ago about rinsing and cleaning in RO and there's no time to bed in again, when I used tapwater, it took a while to start working again. Just a thought.
Also, if you clean with your finger, does the oil from your skin not stop it working for a while too? Or am i being far too anal about something as daft as a skimmer...?