my piaggio zip 50?

hello i have a 51 plate piaggio zip 50 scooter i am having a problem with it. When I try and start it on the button it wants to start but doesn't. I have tried the kick start and that does not work either. I have changed the spark plug and the old one has a black sooty colour on it. I have also filled up the battery acid level and charged the battery and that is taking on a charge and has the correct volts to it. Does anyone have any advice? Will be much appreciated.