pop up sink stopper

[Deeko]

Help! Hi there. I have a sink stopper that doesn't seem to be the normal faucet type with the rod mchanism behind the fixings. This type works by pushing the stopper down to engage/release it. It also seems to have a plastic piston attached in the drain. It now just 'plops' into the hole without any resistance and does not pop up or engage . Hopefully by the images I have attached, somebody could tell me what type of sink stopper it is and how to fix it.

Thanks in advance.

[pushkins]

It should screw down into that white nylon nut in the drain, you screw them down (usually just a few turns) then just press them down to plug the drain and press them again to unplug.