Try to remove and put it back, if that wont work. It has a hardware problem, maybe the heat sinks are not intact very well thus wont make the heat subside.
Are its coolers work property?
If they do,check the case coolers and see if you've got the best airflow you can get.
Sadly,if the removing and putting it back doesn't make better results at last,is rather a hardware problem hardly [or even impossible] to fix by yourself.
If the heatsink isn't very warm that is a bad sign. It may not be in contact with GPU itself.
I would certainly consider an RMA since it is still under warranty. Otherwise you could remove the heatsink and investigate the thermal paste application yourself.