unable to open the mac mini

Hello,

I was just about opening the mac mini to upgrade it but I cant get it open!

I have watched a few instruction videos made about it, I even have the mac_mini servicing pdf from apple in which it's clearly instructed. I used razorthin but wide puttyblades (see picture) on each side starting from the back and trying to push them out to a maximum. Sometimes I hear popping sounds put the case doest lift from it's base by a milimeter...it's FRUSTRATING!

what am I doing wrong?
I did notice I can slide the puttyblades only a few milimiters in untill it hits the inner bottom of the aluminium casing, is that the deepest you can insert them or do you need to work "around the corner" perhaps?



Any help would really be appreciated