I've used a home-made bubbler cloner for years and get a 100% success rate on my clones. Splifman is correct ... the airstones purpose, besides introducing oxygen into the water, is to have bubbles bursting just below the net pots. The solution should be within 1/4 inch from the bottoms of the net pots and the timer 30 on and 60 off is fine.
You can build your own for just a small amount of money. Just go to WalMart and buy:
One small tub with the lid.
One, dual outlet air pump (aquarium dept.)
Two 12" Air Stones (aquarium dept.)
One package of clear air line. (aquarium dept)
One timer (electrical dept.)
Go to the hydro store and buy:
One slab of 1" RockWool cubes.
Twenty 2" net pots.
Drill (with a hole saw) as many 1 7/8 inch holes as you can in the lid.
Drill two small holes in the side of the tub, as close to the top as you can.
Place the AirStones in the bottom of the tub. (You can affix them with double sided tape.)
Run the clear air lines from the pump, through the small holes, then connect them to the Air Stones.
Connect the pump to the timer.
Fill the tup with your solution so that its right under the net pots.
Plug in the timer.
Place clones into the RockWool, then into the net pots, then into the top of the lid.
Keep topping up the solution level.
Wait as long as 10 days to two weeks to see roots. Some will show much sooner than this though.
That's about it ... and you have maybe $30 invested.
Good luck ...
Vi