Devin:
I have a 2007 frontier and it behaved the same way. I was told it probably needed burped. I had to raise the front of the pickup fairly high to get the radiator fill higher than the area where the water gets to the heater to finally get the air out of the system. That fixed that problem for me. Oh! and I left the cap on the radiator and loosened the pressure cap on the overflow bottle to do it. Depends on where your pressure cap is located.
good luck with it.
Ed R., January 2013
