I can't get the bottom cover off

The laptop is off, I've taken all the screws out, but only 3 of the 4 corners will lift. If the laptops front is facing me the bottom left corner just won't come off. I need to clean the fan because it gets really dusty. my macis a 2012 model. I've cleaned a fan on a 2008 model an hour ago and it was so easy to to, but my own laptop just won't budge. I've heard that there are clips that hold it down and that I need to left directly up, but it won't work. Please I really need to clean the fan!