IME, this is the only problem with the TF1000 skimmer. I have one and every once in awhile it will just stop skimming. I have never been able to nail down the cause of it. At first I figured I had done something to alter the surface tension of my water (hand in tank, feeding,etc..) but after 3 days or so of no skimmate I knew there had to be more.
My TF will still produce a thick cloud of micro bubbles, but no matter what I do it still won't produce skimmate. There are a couple of ways to deal with it.
1. I hooked an airpump to my air intake. While this isn't a cure-all, it does seem to decrease the odds of my skimmer going into "nonfoam" mode.
2. If it stops skimmer, I typically disassemble it and scrub it down. Once I have it cleaned, it will start skimming again after an hour or so. The funny thing is that it usually isn't that dirty when I clean it.
3. If it stops producing airbubbles, instead of worrying about the plastic nipple the air hose hooks to, look right inside the air hose itself. Every time I have had to remove a blockage it was inside the airhose, not on the nipple.
I am still running my needlewheel in the RIO 2100. I know that Aquamedic began offering the original pump the skimmer was built with in Europe (is it an Aquabee?) but since I will be replacing mine I haven't upgraded. I also removed the little grey water level control valves on the outlet and only put them in when I need to run a wet foam, such as after my tank upgrade to remove the Southdown snowstorm.
If you do a search, there was a discussion over a year ago about this same problem. If I recall correctly, we all agreed that the only way any of us were able to "restart" the skimmer was to disassemble, clean, and reassemble the whole unit.
HTH,
Scott