Losing brake power

Hi everyone some help on this would be great. I've recently had new pads and discs on the front, before I had new brakes on front the brake performance was crap turned out pads had seized to piston. Anyway after new pads and discs also flushed system with ap racing 5.1 brake fluid the brakes are much better but they seem to bite well from cold then get better when warm then like fade all together like there's no feel in the pedal. The discs are genuine brembo s with mintex mdb1144 pads which I thought was a good combination. Is there a fitting fault ?? Or something not right in the caliper ?? It used to do this before pads were changed.....

Any help appreciated
Thanks in advance ..