Most stoppers are threaded, they look like a little umbrella. The "top" of the stopper has a rubber ring around the circumference that seals in the drain.

The "shaft" of the umbrella/stopper is threaded, it is threaded into a nut on the actuator and the stopper can be rotated to raise or lower how the stopper sets into the nut, that vertical adjustment affects how high the stopper itself sits in the tub's drain.

If your stopper isn't sealing than it could be that your stopper is sitting too high. Even when closed it's not sealing. You could try opening the drain, then rotating it to see it it lowers a bit, then close the drain to see if you get a better seal.

The flip side is when the stopper is threaded too low. It seals fine, but when you open the drain the stopper can only rise a smidge so your drain drains so slowly. In that case you "unscrew it" so it raises up a bit.

That would be the first thing I'd check.

Raise the stopper like you're going to drain the tub. Then with your fingers turn the drain cap clockwise a couple of spins. That should lower it to give you a better seal.

Sometimes there is a cable or lever connecting the drain knob to the drain assembly and that can be out of whack.

We removed the gasket, cleaned it and also adjusted the stopper. It works great now. Thanks Mongo!

Hi. My brand new cable drain has the same issue, I have tried lowering the hex screw in the shaft to its lowest and fill the tub half way and still, it does not hold the water. I am out of ideas.... It is a good trusted brand, the manufacturer sent another stopper that sits inside the opening other than the mushroom/umbrella. Can I convert the drain to some other type of stopper?