Nothing wrong with that one. I've fitted many of them.
I can only think that it is stuck down. There's probably a rubber O-ring inside which does the sealing.
It must surely be binding on the o-ring.
It should pop up a fair distance to let the water out. I guess it's not doing that. When it's up, you should be able to grab it easily to rotate it or unscrew it.
In your position, i would unscrew the whole waste and take it out to inspect. With a traditional pedestal type, you can do that without having to remove the basin from the pedestal.
While we're at it. Are you sure you wanted an "unslotted" one? Pedestal basins usually have an overflow.