Heater Issue

[PFD404]

I jus purchased a 2011 limited 2 months ago and noticed when I set the heater (manual mode) to a temp (i.e. 70) it will blow out hot air at first but 20 mins later it will be blowing out cool air. If i turn the temp up a couple degrees (i.e. 72) it will blow out warm air again but then 5 mins later it will start to blow cool air. On manual mode shouldn't the heater be blowing out a consistently warm air...is something wrong with mine? Thanx in advance