Problem With Pop Up Plug

[Chipchopper]

I have a bathroom pop up plug that keeps popping up and will only stay down if I apply lots of pressure to the actual plug top.
The rod which lifts and lowers the plug seems to be working ok and the nylon attachment, below the plug is reasonably free of gunge.
I've fitted a new washer (thinking that was the problem) but that didn't work either.
The plug is the type that just lifts straight out without having to undo anything.
Any tips will be much appreciated.

[Buenchico]

http://diy.stackexchange.com/questions/8785/why-does-the-pop-up-on-my-sinks-drain-keep-popping-up

[Chipchopper]

Thanks Bueno, thats a good link I'll be looking into it. Cheers

[The Builder]

Awful things, Chippers. I'm about to replace mine with something else. Chris has it covered. They're easy to adjust underneath.