My Old Cooler Wont Come Off!

I bought a pc around 6 months ago, and now im changing the cooler from a Cooler master 212 hyper, to a Arctic Liquid freezer pro III.

And as im removing parts, the cooler wont come off!

And i have never done this before so I have no clue about what im doing.

So to put it simply, im turning the screws of the bracket but they wont turn, and my friend said that the back plate was interfering?