I've had issues with tuning my skimmer as well, especially after a water change. lol I have no idea why, the water level stays the same, I even marked it with a sticky so i would know where the level was when it was working.
I just today got mine back to normal after a few days of tweaking.
For reference I have an eshopps psk 150. Not the most expensive skimmer but not the cheapest either. lol When it is tuned just right it works like crazy though. Just emptied a cup of goo a few minutes ago.