Remove the HSF. Thoroughly degrease it and the core. Re-apply AS and try again. It only takes a speck of dust, small enough to nestle in a crease in your hand, to fall on the core and that alone can screw up the mating of the two. Last thing I do before settling the HSF on the CPU is inspect both mating surfaces with a 10x jewellers loupe. It would amaze you how small crud can appear out of nowhere and find its way onto the core surface during handling.