Toilets, Sinks, Showers, Tubs and Disposals - Drain wont stay plugged

Hello,
We have a drain in one of our bathrooms of the type that you turn it one direction to have it lower and the drain should now be plugged. Well i can't seem to get it to stay plugged. I have tried removing the screw allowing me to repositioning it but it still seems to leak water although at a slower pace then before having messed with it.

What is the best method to get this drain to plug so we can bath my son without having to turn the water on every few minutes to refill it.

Thanks for the help.

Home improvement stores (Lowes, Home Depot, Ace Hardware, True Value...) sell replacement tub stoppers. I have had good luck with the type that screw into the drain and are spring loaded to pop up/down. They have a rubber flange similar to a suction cup that seals well with the drain of the tub. You can also get tub/sink drain stops that are nothing more than a round sheet of rubber. Lay it over the drain and the water pressure holds them in place. Sometimes they work better with the old drain stopper removed.