Hi
I would up the length of time on the naughty seat, probably only have to do it once or twice for the effect to take place. She has worked out that 2 minutes is not that long and worth it - she may not like 4/5 minutes and will get upset at the sudden change of rule. After all we make the rules and we can adapt them up to suit the behaviour. Once the naughty seat starts to work you couild then reduce it to the age she is - 2years = 2 minutes etc
Hope this helps and good luck - the naughty seat still works for my kids - 9 minutes sounds an eternity to my daughter and the fear of it is warning enough to stop the behaviour.
Fiona