Renault megane-Whats wrong with my handbrake?

I have a 07 2 ltr Renault coupe/cabrilet with an aircraft type brake handle. It sometimes will not release when driving stop/start in traffic jams. I press release button but doesn't release. Have to pull brake on harder and harder with button pressed and eventually releases. Very annoying for waiting traffic behind and worrying incase it wont release at all.