How can I get my platforms to randomly appear when not activated by button

So I'm building a game and the first puzzle is a jumping puzzle. I have already created a button collision so that when the player enters the trigger the platforms will appear for 8 seconds. However, when the button is not on I want my platforms to all appear at a random rate so if the player wants they can figure out the jumping puzzle without pressing the button. I have tried InvokeRepeating them so it happens every 0.5 seconds and it says "invoke method couldn't be called" I have also tried a coroutine and that doesn't work properly either. Any help would be appreciated :) Thanks :)![alt text][1] [1]: /storage/temp/55834-untitled.png