what i think is the part where it clamps the pads onto the rotors.. forgot whats it called.. but it has a rubbery cover over on it and when i had that problem.. i put some brake lubicration on it and it grabs wonders.

also you might want to double check if each rotor disc has 2 pads.. some ppl just replace the outer one and not the inner one. seen that happen and its not a pretty site.