If you fail one of the events such a pool party or wedding, it will then take you back a step.
Example: I have not completed a hard event in the time provided, the next event I will be able to play for this type of party would be a medium. Again if I failed the medium event, it would take me back to an easy one.
You can fast track this by starting and events and pressing the end events button, once you confirm this for you next event it will take you back down a difficulty.