Make sure that the cistern is completely full (there is usually a level mark to be found somewhere). Some flush units also have a plug in them which restricts the volume of water that can syphon - make sure the plug is there, if applicable. You can adjust the volume of water in the cistern by adjusting the fill valve, often enough.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.