Are you surfing on cell data? Cos it won't change while you're on wifi. It only monitors gprs.

If you are, you might want to check how often it's updating. Open the homescreen settings (little cog top right) and your widgets and shortcuts all get little Xs so you can delete any of them on top right. On the top left of data monitor is a tiny spanner. Click that ans check the monitor settings. Mine updates every 5 seconds.