Are you sure the processor is making good contact with the heatsink? Sometimes it's just not making proper contact, as simple as that may sound. The best way to check would be to clean the heatsink/processor - apply new stuff, and then put the heatsink on. Taking it off and examining the area on the heatsink for areas of application in comparison to the processor. If it is indeed making good contact, then it's likely inadequate airflow maybe caused by the fan itself. The fan that came with it should have no issues heating it and unless there's something truly wrong with the heatsink itself (If it is indeed making perfect contact) that would likely be related to the fan itself, rather than the heatsink. Unfortunately if that's the case, the best thing to do is purchase another one.